Argumentum Ad Hominem is a logical fallacy.
Liberals telling the truth is a fantasy.
The political leanings of the snopes couple is not an Ad Hominem argument. It is valid because it is part of the evidence demonstrating that snopes has a left leaning agenda and that agenda often colors what they publish as “facts”.
Now, if I were to say that either of them is ugly or smells bad or stutters and that makes their info suspect, that would indeed be Ad Hominem.
Noting their demonstrably leftish bent is not a “personal attack”. It is a relevant point.