This is about as silly an argument as I have ever encountered in Shakespearean scholarship — even sillier than the celebrated Impediment of Adipose.
Jonson's compliment is a fairly pretty one: “Despite your lack of a Classical Education (like Mine), your work commands the admiration of the Classical Masters.”
But the reading Ingleby urges makes no sense at all: “Even if you lacked a Classical Education (like Mine), your work would command the admiration of the Classical Masters — all the more impressive, then, that you achieved this in full possession of a Classical Education.”
Jonson was an arrogant and hot-tempered SOB, but he was not an imbecile.
I think there is a clue to this just a little farther in the passage itself, where the speaker goes on to mention his "state"... and I think it is in the notion of "statehood" that these expressions (and perhaps their understanding) are to be grounded.
That is, the speaker is speaking from the state of not just being a man, but man, as in ALL men, and further, ALL (of) "man", that is, a state of mind not just identifying with all the members of that given set, but to represent them singularly, collectively, and superlatively (cf. expressions like "I am woman, hear me roar!").
Additionally, there is an allusion to "man" being used here as an abstraction, not just a concrete representative of same, or all of them, but the whole idea of being a man (or woman, later), as in expressions like "I am VENGEANCE," or similar identifications with abstractions of concrete things, or abstract concepts themselves. This identification-with-the-concept is what gives the speaker the presumed mandate to speak on that entire concept's behalf, as he indeed goes on to do throughout the quoted passage.
However, as the somewhat more modern examples above suggest, I am not sure this is so much a feature of Shakespearean English, but rather, that Shakespeare's prose simply uses more of this forceful stuff. Indeed, I would bet you a shirt frill that if H.G. Wells dumped you in ye local publick house in Merry Ole Stratford-Upon-Avon, you'd be hard pressed to hear any of the locals express such prose while quaffing their brews.
But that, of course, is pure speculation on my part. :o)
Best Answer
Because the original sense of the word in English, per the OED, was:
Which also includes the following citation: