The distinction he makes between modern and ancient biographies is that the latter is less concerned with a factual chronology of a life than with capturing the character of an individual.
Is this the case with the authors I mentioned above?
Edit: Since there is a good chance that the individuals I mention above are contemporaries of the Gospel authors, are there other stylistic traits the Gospels might share with writings like Parallel Lives? Or would the likely geographic separation of these people prohibit that?
Where is he drawing this comparison from? Suetonius, Plutarch, Tacitus, etc?
In The New Testament: A Historical Introduction Ehrman explicitly makes the comparison to the Life of Apollonius of Tyana by Philostratus. The second chapter goes into detail regarding the shared characteristics Ehrman thinks are important as part of a discussion of genre analysis. Given that, I'm sure he meant the comparison more broadly but this is the explicit example he uses.
I don't know the specifics of Ehrman's comments, so I don't know how apt the comparison is. It's not a shocking idea; the Gospels are Greek biographies in the sense they are written in Greek and describe the life of a person. I've seen an argument made that Gospels are some sort of response to Epic, so people are trying to find a genre in contemporary literature to stick them with; the argument for biography could be reasonable.
Here is something from the beginning of Plutarch's life of Alexander:
For it is not Histories that I am writing, but Lives; and in the most illustrious deeds there is not always a manifestation of virtue or vice, nay, a slight thing like a phrase or a jest often makes a greater revelation of character than battles when thousands fall, or the greatest armaments, or sieges of cities. Accordingly, just as painters get the likenesses in their portraits from the face and the expression of the eyes, wherein the character shows itself, but make very little account of the other parts of the body, so I must be permitted to devote myself rather to the signs of the soul in men, and by means of these to portray the life of each, leaving to others the description of their great contests.
So there you go. Plutarch's completely aware he doesn't deal in The Facts and he doesn't pretend to be. He wants to tell you whether this guy is a good guy or a bad guy, and he'll use all the unverifiable stories he needs to get you to think what he wants you to think.
This is generally taken as the rule for ancient biography. I don't know anyone else that sets it out as clearly as this, and just because Plutarch is doing this doesn't mean every ancient biographer is doing this. The OCD includes under the heading "biography, Greek" genres ranging from funeral dirges to the biographical novel and suggests
a complicated picture of overlapping traditions, embracing works of varying form, style, length, and truthfulness.
The OCD does mention the Gospels in this entry, noting their "charismatic hero and their anecdotal narrative," and compares this to Plutarch (we just saw Plutarch himself say he was interested in talking about virtue and vice and the little things that happened at dinner), with Plutarch getting the better end of the comparison.
With the proviso that we remember genres are not solid as the OCD reminds us, and that Plutarch is just this guy, you know? he was a master of his craft and if he says he's more interested in anecdotes than every damn battle, it's probably fine to accept this as one at least understanding of what Greek biography was supposed to do.
Without getting into questions of what exactly truth is anyway, I don't think it would shock anyone to hear that ancient authors were less interested than we are in getting down every little thing right.