Actually, I have a view that's slightly more favorable to trump on a point that I haven't seen anyone else say.
The CONTEXT for the comment was Chris Wallace not only asking trump to condemn groups, but to ask them to 'stand down'.
That led to trump - who initially interrupted Wallace after the 'condemn' part to say 'sure' -seizing on the 'stand down' phrase.
In the way he does when he 'has to say something', almost sing-song, he realized he had to mouth the words, with obvious insincerity, which he showed by pretending he couldn't even think of any and asking for what name 'they wanted him to say'. Biden said Proud boys.
Then trump said 'proud boys, stand back and stand by'.
Now, a nefarious interpretation of that, and a common one, is that trump was saying 'stand by' as in 'you are my forces, wait for the go signal to attack'. And that interpretation IMO does fit trump's values.
But based on unfortunately watching him speak, I suspect he's not that able to come up with a phrase like that on the spot intentionally.
Just as he misspoke the phrase "herd immunity" as "herd mentality" when he grasped for it.
trump's "stand by" came out to me like just his fried brain grasping, a distortion of Wallace's "stand down" just as trump messed that up to "stand back" also; that he was in 'surrender', mouth the words they want mode, not 'pick a fight by openly defending white supremacists' mode. He wasn't willing to say he condemned them, but he'd already said 'sure' that he was willing to, and I suspect this was just his garbling the insincere words.