That's how I learned about it.
It is impossible to counter each lie, especially in a debate format, because there are so many that come so rapidly. They are so outrageous that they have the effect of leaving the interviewer, or debate opponent in this case, so stunned that they search for a comeback while the next several lies are still spewing.
For someone with a stutter, that is very problematic. They trip over their own words and lose their train of thought before they can respond. Add to that some fatigue, not just from the European travel, but from a busy schedule after returning home. Add to that a cold. (And maybe cold medicine? )
In 2020, Trump's aggressive demeanor in the debates was intended to throw Biden into a stutter that would make him look bad. It failed because Trump just came across as the clod that he is and Biden kept his cool.
But this time around, I am 100% certain that Trump was coached on the gish gallop prior to the debate. Yeah, he lied when he said that he did not prep for it.
In the normal business of a president, a gish galloping person is not a problem. He can just walk away. He will have advisors with him who can join him in rebutting the fast and furious lies. He can just say, "I will not engage in this kind of nonsense. Come back when you are ready to talk sense." Boom. End of problem.
But a debate candidate cannot do that. He has to respond with 1 and 2 minute limits to a slew of outrageous lies that would take a whole day to put down. And he also has to present his own positive, contructive views and policies.
So, I have zero concerns about Biden's cognitive abilities. Considering what he was dealing with in Trump, the constraints of the debate setting, his cold and being tired from his schedule, Biden did as well as anyone else in the same situation would have done.