I fail to see how this is an ad hominem, can you please explain?

It’s the “ad hominem fallacy fallacy”—in a nutshell, thinking that abuse (or mere criticism, in this case) is an ad hominem attack.

I love this article on the subject: https://laurencetennant.com/bonds/adhominem.html

Ironically, the GP is ad hominem; as the above link says, “while he pretends to deal with A's argument, in using the term "ad hominem" incorrectly, B is in fact trying to dismiss the argument by imputing that A is resorting to personal attacks.”

Kind of ironic to invoke a dead language in criticizing this essay, given its content.