I see your point, but I think it’s a pretty subtle distinction.
I’ve always understood an ad hominem to mean you are discrediting someone’s ideas based on personal attacks on that person rather than the merits of the idea itself.
But I was never taught that ad hominem must solely be rated to a persons character (like your cat example), but it could also be on errors or mistakes they’ve made in the past which call into question their current abilities.
I do think it’s illogical to say “because you failed to predict last year’s Oscar winner, your predictions about dark matter are worthless”.
That said, in this case the OP’s post seems to be related to retroactively questioning previous good work due to issues now. I’m not sure if that time sequence makes a difference in this case.
This form of the argumentum ex concessis occurs when someone is accused of being hypocritical and is personally accused of not believing in, or not acting in accordance with, commitments he has taken on another occasion.
Doesn’t it put him directly at odds with core anarchist principles, specifically opposition to all involuntary coercive forms of hierarchy (like the conscripted armies on both sides) and the existence of states, which all of the Western powers were? I don’t think it’s unreasonable to say “I don’t accept a man who openly rejects the core principles of anarchism as an authoritative source on anarchism”.