Seems like an unpopular opinion, but after watching the clip 3 times, seems to me that Trump wasn't telling Proud Boys to "stand by" in the sense of permitting them to act, but that he was being a senile dotard trying to tell them to stand down as requested.
I think this is contextually clear from his preceding responses to Wallace asking him to condemn such groups: "Sure," "Sure I'm willing to do that" "I'm willing to do anything" "Give me a name. Who would you like me to condemn?" "Proud boys?"
He's inarticulate + senile + seemed like he was trying to rush through answering so he could change the subject back to Antifa and the left, and that he just stumbled in the process as he often does.