Ad hominem is not always a fallacy. This is an actual, material conflict of interest that the government has (which happens in all of tax law). The existence of that conflict does discount the validity of the sources somewhat, but it is an exercise for the reader to decide how much.
Personally, I'm not so convinced by the 5th amendment vs tax returns argument, but that's not because a bunch of government lawyers are telling me that the 5th amendment doesn't apply.
