I think in the debates 4 years ago, he was trying to attack Biden and get him flustered, but that didn't work well. He has had some time to think about it.
I think he did get Biden flustered, but people were already used to seeing Biden angry and insulting people, so it didn't faze anyone. For some reason, it takes a lot of anger and viciousness from Joe Biden to even get Republicans to see through the media's "nice guy Joe Biden" mythology.
It's also funny how Leftists are allowed to be (often encouraged to be) righteously indignant, but angry righties are dangerously unhinged Angry White Men who need their guns taken away. But that's another topic.