Trump's folks calculated how to deploy Biden's speech impediment to their advantage - the way Trump rattled off lies gave no chance to respond to each in turn, there were too many. This was a debating technique called the Gish Gallop.

This from historian Heather Cox Richardson:

Trump also directly accused Biden of his own failings and claimed Biden’s own strengths, saying, for example, that Biden, who has enacted the most sweeping legislation of any president since at least Lyndon Johnson, couldn’t get anything done while he, who accomplished only tax cuts, was more effective. He responded to the calling out of his own criminal convictions by saying that Biden “could be a convicted felon,” and falsely stating: “This man is a criminal.” And, repeatedly, Trump called America a “failing nation” and described it as a hellscape.

It went on and on, and that was the point. This was not a debate. It was Trump using a technique that actually has a formal name, the Gish gallop, although I suspect he comes by it naturally. It’s a rhetorical technique in which someone throws out a fast string of lies, non-sequiturs, and specious arguments, so many that it is impossible to fact-check or rebut them in the amount of time it took to say them. Trying to figure out how to respond makes the opponent look confused, because they don’t know where to start grappling with the flood that has just hit them.

It is a form of gaslighting, and it is especially effective on someone with a stutter, as Biden has. It is similar to what Trump did to Biden during a debate in 2020. In that case, though, the lack of muting on the mics left Biden simply saying: “Will you shut up, man?” a comment that resonated with the audience. Giving Biden the enforced space to answer by killing the mic of the person not speaking tonight actually made the technique more effective.

There are ways to combat the Gish gallop—by calling it out for what it is, among other ways—but Biden retreated to trying to give the three pieces of evidence that established his own credentials on the point at hand. His command of those points was notable, but the difference between how he sounded at the debate and how he sounded on stage at a rally in Raleigh, North Carolina, just an hour afterward suggested that the technique worked on him.

It wasn't a real debate, there was no real-time fact checking, they didn't do anything to keep Trump on topic.

Biden doesn't need to ever talk to Trump in person again, and it seems that this gaslighting adventure is another reason why.