I agree that "stand by" does sound problematic, but I would let is slide as a slip up, given that the moderator asked whether he will tell them to "stand down" -- it's an easy mistake to make (and talking about slip ups, Joe Biden has more than enough of them as well).
You can see Donald Trump denouncing Proud Boys here: https://www.youtube.com/watch?v=Bd0cMmBvqWc&feature=youtu.be...