Marx explains why he's quoting Sismondi. He believes Sismondi's comment shows that the Roman class struggle and the modern (of his time) class struggle are so totally different that they cannot be compared. While I must remain sympathetic to that argument (i.e. that the two cannot easily be compared), Sismondi's comment is simply mistaken. It was common in the Enlightenment and especially in the Victorian Period to characterize the Roman plebs as the "idle plebs," I believe a term coined by Mommsen (he at least used it). That is, the prevailing view of the urban plebs was of an idle population that did little but eat up state-subsidized grain and watch arena shows. In this view, the state supported the urban plebs through grain distributions (made free by the lex Clodia of 58) and the slaves did all the work. Marx buys into this exact view when he says "the ancient Roman class struggle was only fought out within a privileged minority, between the free rich and the free poor, while the great productive mass of the population, the slaves, formed a purely passive pedestal for the combatants." In other words, thought Marx (who sometimes has a rather loose understanding of "proletariat" in the Roman world, believing it simultaneously to mean the plebeian order and the plebs, two very different things) and most of his learned contemporaries, the plebs basically spent their time doing nothing and were supported by largess from the state.

This view, rooted as it is in the grain distributions, is wholly erroneous and has been completely abandoned by any serious scholar of the Roman world. For one thing, the grain distributions, while nice, were not sufficient for a family to survive on, and under Augustus were not given to the most needy. For another, the stabilization of the grain market in the city was not a privilege, it was a need. Rome did not have a hinterland capable of feeding it, and the city's population was maintained artificially by massive imports of food. If disturbed, by storms or famine, or by human action such as the hoarding of grain by private traders to hike up prices by creating artificial scarcity, the city was in quite literal danger of starving to death. I have a number of threads scattered through this subreddit in which I explain precisely while the so-called "grain dole" was nothing like most people imagine it. This one will do as well as any other, I suppose, along with its associated link. If you were to search either here or on Google you'd find more, I keep misplacing mine and always have to rewrite everything from scratch like a dingaling. Also of importance are this thread, on the grain supply, and this one, or more precisely the thread therein linked, which expound on the monetary difficulties of the urban poor, who were predominantly semi-itinerary day workers whose lives were stretched very thin and whose wages were hardly enough to provide for themselves--the urban plebs had to work all the time to survive. Excellent reading on the subject may be found in Garnsey's Famine and Food Supply in the Graeco-Roman World (1989), Rickman's The Corn Supply of Ancient Rome (1980), the relevant portions Harrison's article Catiline, Clodius, and Popular Politics at Rome During the 60s and 50s BCE (2008, really not about this topic, but provides one of the most succinct overviews of the lives of the urban poor I've ever seen), Brunt's The Roman Mob (1966, old but the basis for the "modern" view of the urban poor), and Purcell's chapter in the Cambridge Ancient History, entitled the City of Rome and the Plebs Urbana in the Late Republic (though I must disagree with many of his comments about collegia)