Hoc non solum est X, verum etiam Y: domum esse familiarissimam . . .
hoc est bonum: me venisse . . . – "this is a good thing: the fact that I came . . . "
The acc. + infinitive clause is in apposition to hoc. It explains hoc, and maybe the sentence should be punctuated with a colon instead of a comma after est.
“But this, too, was not just [characteristic] of his literary brilliance but also of his upright character: [namely, the fact] that the same family that fostered him in his youth was on intimate terms with him in his old age.”
I think this makes sense; but, like you, some editors have found this passage troublesome and attempted to remove the perceived difficulty by offering proposals to emend it. What was a little troublesome–to me, at least, but in the end not fatal–was not the acc, + infinitive, but the genitives depending on est: “this is of his literary brilliance”, i.e., “this is characteristic of his literary brilliance” or “this is indicative of his literary brilliance.” I translated the acc. + inf. construction by punctuating with a colon after est and inserting “namely, the fact that” to show how it is in apposition to hoc.
Leaving aside the question of what the manuscripts have and what is conjecture (I suspect the acc.&inf. is the latter): as Hylander indicates, hoc est bonum, me venisse is really no more strange than bonum est me venisse. (Either one means “It’s a good thing I came.”) The deictic hoc simply points forward to the acc.&inf. noun phrase; Latin, like Greek, likes to signpost (cf. e.g. ita … ut, οὕτως … ὥστε), and if the acc.&inf. is long or complex you’re more likely than not to have hoc vel sim. introducing it. Compare Greek use of τουτο etc., e.g. τουτο (εστι) καλον, οτι …., or τουτο ειπον, οτι …. The acc.&inf. can be the subject or object of a sentence, whether or not picking up a hoc (in which case it’s technically in apposition, but that’s just grammarian-speak). It doesn’t have to be ind.statement, though of course it can be: dixi me venisse or hoc dixi, me venisse.
I think a simple comma is better than a colon myself, since the construction is continuous, and that’s what editors normally print. But punctuation is not a very scientific business (except of course for Germans).
Another discrepancy between Henle and Perseus: The entire passage in cursive is missing in the Perseus’ version. Why? Does Perseus generally have the latest ‘up to date’ versions of these texts?
[9] an domicilium Romae non habuit is qui tot annis ante civitatem datam sedem omnium rerum ac fortunarum suarum Romae conlocavit? an non est professus? immo vero eis tabulis professus quae solae ex illa professione conlegioque praetorum obtinent publicarum tabularum auctoritatem. Nam quum Appii tabulae negligentius asservatae dicerentur, Gabinii, quandiu incolumis fuit, levitas, post damnationem calamitas, omnem tabularum fidem resignasset, Metellus, homo sanctissimus modestissimusque omnium, tanta diligentia fuit, ut ad L. Lentulum praetorem et ad judices venerit, et unius nominis litura se commotum esse dixerit. His igitur tabulis nullam lituram in nomen A. Licinii videtis.
I think Perseus uses texts that are no longer in copyright. One often notices discrepancies in the passus numbering of texts, which means it may take a little while to find the wanted locus.
In Cerutti’s student edition (after Clark’s Oxford edition) this passage is strikingly different in many details:
“Nam cum Appi tabulae neglegentius adservatae dicerentur, Gabini, quam diu — — in nomine A. Licini videtis.”
Perseus’s text is supposed to be Clark’s OCT, but obviously it isn’t entirely reliable.
The differences between Clark and Henle are mainly spelling differences, except for nomen/nomine, which are variant readings found in different manuscripts.