1. Clinton neither admitted nor denied it. She only said she "didn't recall" making that statement.
2. In any case, Clinton has been very openly critical of Assange, saying the charges were not punishing journalism and that "he has to answer for what he's done." [1]
Ad hominem is a fallacy if you are arguing hypotheticals and philosophy in a Greek salon.
In understanding how the world around us works, credibility matters quite a bit, and "I'm not interested in pretending True Pundit says true things" is a pretty reasonable shortcut.
Rather than just thought-terminate with "logical fallacy," the burden is now on the one bringing the evidence to bring it via a channel other than True Pundit.
Clinton has had a knack for knowing the real truth of a situation and either not wanting to share that with the public, or doing it in a haughty way where she's simply not believed. Knowing what I know about her in that way, such a quote is worrying.
It implies that she's being characteristically tonedeaf and screwing up the communication of some pretty serious concerns about Assange, but I think that's no mystery by now. You can always get Clinton to make it all about her and spin it in a way that can let you get away with damn near anything, but that's just exploiting personal failings on her part, where if you dig into what she knows it's unsettling how sharp she is.
You can't go by whether Clinton's screwed up the optics.
2. In any case, Clinton has been very openly critical of Assange, saying the charges were not punishing journalism and that "he has to answer for what he's done." [1]
[1] https://youtu.be/Qc19Qk3KKCw?t=50