Saying you didn't expect a good argument from a source due to reputation is not an ad hominem attack.

Their expectation even had a reasonable explanation: their previous encounters with the source (as they saw them).

Then their actual judgment was on the merits.

And even their credibility expectations were based on actual experience (as they interpreted those) with the target, not something irrelevant to credibility.

It is a good example of someone whose experiences might have prompted them to make an ad hominem attack, but who didn't. The opposite of making such an attack if there is one.

When someone prejudged a claim based on who made it, that’s an ad hominem.

Kudos that OP actually read it, but that doesn’t change “I thought the argument was weak because of who made it”