It is only very rarely that we enter non-eggcorns into the database, but I am making an exception for ad homonym.
First of all, homonyms — or rather, homophones, i.e. words that sound alike but aren’t necessarily spelled alike — enter into the genesis of eggcorns themselves.
Second, because the ad homonym malapropism illustrates very nicely what elements are required to make an eggcorn: it is a non-standard reshaping of an established term (check!), homonym and hominem are pronounced nearly the same (check!), but it isn’t a re-interpretation that is based on (a correct understanding of the semantics of) the target word homonym.
In a typical eggcorn, the writer understands the sense of the word he or she actually employs; the problem is that the use takes up the place already occupied by a different word, often part of a set phrase. Here, however, the eggcorn users don’t give any sign that they know what a homonym is. In one of the examples, the writer obviously believes that ad homonym means against the man in Latin. It’s the Latin that is faulty, along with the recollection of what the expression is supposed to be, precisely. (And spell-checkers might have had their bit to add, too. Case in point: the spell-checker I just used on this entry didn’t know hominem and suggested hominid. Ad hominid also yields over a hundred Google hits, compared to several thousand for ad homonym(s).)
The replacement of a “complicated phrase” by another “complicated phrase” is rarely an eggcorn: often, the writer is unclear about the meaning of both, not only about the original.