Trump said after the debate he hadn't heard of the "Proud Boys". During the debate Trump paused and said "what do you want me to call them" and when Wallace and Biden both simultaneously blurted out "White supremacists and Proud Boys" and "Proud Boys", respectively, Trump went with Proud Boys. I take this new context to mean Trump definitely didn't know what the Proud Boys were and thought that was another term for white supremacists which he then told to "Stand down and standby" during the debate.