I kinda see what you are saying. In a perfect world Trump should have just responded. "I denounce White Supremacy" but in reality he's probably tired of having to that shit. He's done it before and the implication of Wallace's comment was that he hadn't. Biden and Wallace were teaming up on him and he got flustered. Even still, he did condemn them. When asked if he would condemn them he said "sure" and then told the Proud Boys to "stand down" (albeit adding on the "stand by" which really just him misspeaking because everyone was talking at once). Trump is tired of the implication that he is a racist and in true Trump form he's not going to just bow to the whims of Chris Wallace and play along with his charged questioning. Trump has to make his answer interesting. Any press at all, is good press. And either way, Trump was going to get bad press? He straight up condemned it at Charlottesville and to this day the media just lies about it. If Trump just straight up says "I condemn White Supremacy" they would just not cover it and lie about it. This way they actual play his comments and the American people can see for themselves that the media is twisting the narrative against him.