Here is the passage from the book
It is only in the first century BCE that we can start to explore
Rome, close up and in vivid detail, through contemporary eyes.
An extraordinary wealth of words survives from this period:
from private letters to public speeches, from philosophy to
poetry – epic and erotic, scholarly and straight from the street.
Thanks to all this, we can still follow the day-to-day wheeling
and dealing of Rome’s political grandees. We can eavesdrop on
their bargaining and their trade-offs and glimpse their
back-stabbing, metaphorical and literal. We can even get a taste
of their private lives: their marital tiffs, their cash-flow problems,
their grief at the death of beloved children, or occasionally of
their beloved slaves. There is no earlier period in the history of
the West that it is possible to get to know quite so well or so
intimately (we have nothing like such rich and varied evidence
from classical Athens). It is not for more than a millennium, in
the world of Renaissance Florence, that we find any other place
that we can know in such detail again.
How accurate is this assertion?
I think first off it is important to highlight her qualification--In the West. Without that this is a very short conversation, with China obviously having a historical output vastly exceeding that of Rome's after the Han Dynasty. I am on somewhat shakier footing in other areas, but I suspect that Abbasid Baghdad and the courtly world of Heian Japan are also more granular in their detail than Rome. And for earlier periods (and here her use of the phrase "the west" becomes problematic) the lucky survival of documents from the early second millennium trade city of Kanesh and the New Kingdom Egyptian worker's village of Deir el-Medina have given us a far more complete picture of life in those places than we get from anywhere in Rome.
Even within the "west" though I find it a bit difficult to see how she came by her description. There is a degree of overstatement in that passage--for example, when talking about "private letters" we are pretty much just taking about one person--Cicero--and to a lesser extent Pliny the Younger, who wrote over a century later. And regarding "the street", while we have some very vivid descriptions of the street from writers like Juvenal and Martial, that is a very different thing from getting voices from the street. On the other hand, there are sources that come into focus during the High Middle Ages that barely exist in the Roman world. Charters, land grants, and court records let us know about certain basic facts of daily life in that period that we do not for Roman equivalents. There are also some extraordinary moments that produce glimpses into beliefs, such as the records of the Inquisition after the Albigensian Crusade that may be hazy and distorted but are nonetheless real. These may not be as lively as a poem about street life but they tell us a great deal more.
What I think she may be referring to is that Rome had a strata of highly literary urban, secular social elite that did not really exist again until the late Middle Ages. Medieval Europe did have literary and cultural "scenes", most famously the troubadours and the chivalric romances, but to a modern audiences these often feel less vivid and more artificial than the earthiness of Catullus and the urbane sophistication of Ovid--it would be easy to argue that no picture of the court of Eleanor of Aquitaine is as lively as Trimalchio's dinner party (on the other hand more troubadours have works surviving than Roman poets so it is bit half dozen of one, six of the other).
The upshot of this is that it is often easier for a modern audience to see themselves in the letters of Cicero than in the writings of Gerald of Wales, relate to the problems facing Juvenal more than those of Hugh of Lusignan, and feel a deep emotional connection to Catullus than to Bernart de Ventadorn. Rome is a very easy society for us to picture, particular the Rome described by the urban literary elite, but that does not mean we actually know more about it than we do a German city with surviving records of property ownership, court cases, guild and labor organization, and market regulations.
I want to preface by saying that I actually like the image of Rome that Mary Beard depicts in SPQR. But I think that the understanding of "the Renaissance", and contemporary developments in medieval studies displayed here is typical of classicists of Mary Beard's generation.
As someone who spent much of undergrad studying the Romans before completing a PhD in the Middle Ages, the sheer level of detail that can be known about medieval society based on extant texts dwarfs late Roman republic.
I love the late republican period as a period of history. But as an example of something that is virtually impossible to do for the late republican period of Roman history can be found in Women's Networks in Medieval France: Gender and Community in Montpellier, 1300-1350 by Kathryn Reyerson. I'm quoting the back of the book, which serves my larger point
"To do this, Kathryn L. Reyerson focuses specifically on the experiences of Agnes de Bossones, widow of a changer of the mercantile elite of Montpellier. Agnes was a real estate mogul and a patron of philanthropic institutions that permitted lower strata women to survive and thrive in a mature urban economy of the period before 1350. Notably, Montpellier was a large urban center in southern France. Linkages stretched horizontally and vertically in this robust urban environment, mitigating the restrictions of patriarchy and the constraints of gender. "
This sort of level of detail found about a single woman in the pre-plague Europe is startling if you consider the corpus of inscriptions and texts copied by medieval Europeans. And this is a small network. Commercial Agreements and Social Dynamics in Medieval Genoa by Quentin van Dooselaere takes a much larger corpus of commercial investments to detail wide-ranging social dynamics in medieval Genoa.
And this sort of network history is fairly dry and reduces humans to words and numbers on a page. But if we take Cicero as our 5-star contributor to the corpus of late republican Latin and compare him to someone who was equally important to their time as both a Latin stylist and leading politician--if we take ecclesiastical politics as a form of politics, but the sort of secular ecclesiastical divide was unknown to Cicero--the extant texts of Bernard far outweigh Cicero's based on sheer volume. We have 37 letters of Cicero. We have 547 letters of Bernard of Clairvaux, and Bernard was twice as sanctimonious as Cicero. This is to say nothing of his sermons and larger philosophical-theological treatises. IF you wish to know the mind of a premodern individual, Bernard is a far more "complete individual" as far as written testimony.
To return to Rome, we have many lengthy historical works from the Augustan age, which immediately follows the late republican period. But we also have numerous historical writings from medieval Italy, which returns me to the Florentine "Renaissance" of the excerpt from Beard. The "Renaissance" is an intellectual movement, not a chronological period, and to be sure, there is a wealth of material from Florence post-1300. But consider the War of the Sicilian Vespers at the end of the thirteenth century--a multi-state conflict contained in much of the same geography of the old Roman Italian heartland. We have multiple narrative accounts of this conflict, including one Sicilian-language account, Lu rebellamentu di Sichilia, Bartolomeo da Neocastro's Latin chronicle on the war, and Giovanni Villani devotes significant attention to the war, especially as it pertains to the Florentine contribution in The Greater Chronicle. This is to say nothing of the ancillary references found in smaller chronicles, like the multi-century Genoese civic Chronicle. It is also worth noting that for Genoa's contributions, their civic chronicle not only references that city's contributions to the War of the Sicilian Vespers, but the Liber Iurium in two (or 3?) massive volumes records all treaties signed by Genoa, including treaties to end the War of the Vespers. There are many, many other ways to find Vespers history through text reprinted and found on archive.org. While there are no characters perhaps equal to Caesar and his firsthand account of his toppling of the the optimates faction, the war of the Sicilian Vespers is documented by such a greater variety of perspectives. And for characters, check out Roger of Lauria, the son of a lesser-italian noble who fled the Kingdom of Sicily when the Angevins moved in after the death of Frederick II with no heir. He was invested with an admiralty and won a series of stunning naval victories, found in a number of textual sources, both narrative and legal.
Apologies for the overlong answer, but I think that it deserves to be said that Mary Beard, for all of her manifold merits, seems to either ignore the decades of medieval studies that happened during her time as a classicist. And I cannot really fault her for that; I have known many a classicist like her. But the level of detail in the historical record for the late Roman republic is dwarfed by the political, economic, legal, etc level of detail that remains in physical form for dozens of urban and political centers in the period from 800-1300 in Europe alone, that is to say nothing of the wider Mediterranean world and Eurasia where the textual record starts to expand exponentially in the post-Roman period.