The appeal to authority is basically the opposite of the ad hominem fallacy.

It is the acceptance of a claim because of the credentials of the person who said it.

It’s a reasonable heuristic for deciding which claims to trust, but it is not a substitute for logical argumentation.

This man who says he’s an authority — and may actually be — has still not provided an argument about why we should consider this a Nazi salute.

Therefore it is fallacious to believe it to have been “proven”.

Again trusting authority is a decent heuristic, but it’s not a source of certainty.

If this expert were to provide some evidence and reasoning, then it would be less fallacious to consider the question closed based on his testimony.