Excellent point, i'll bet many, if not most, have left that out of their thinking. The stutter would be in his mind and no wonder with the strange setting and the fact he hadn't debated for four years, and the fact he was trying to pick detail to deal with the generalized lies Trump was spewing, no wonder he got himself frazzled. He would never be like that sitting with world leaders. And Trump couldn't get away with lying so much to world leaders either.
Of course Trump would look more settled, he didn't care at all what he was saying. It's
easy to be settled if you don't care at all about the accuracy of what you are saying.
When Trump repeated the lie about migrants being crooks, rapists and murderers and Biden tried to talk about a specific case, as Swalwell put it so well, Joe should have just looked at him and said, 'You are lying again. You know research says that refugees as a group commit less crime than others.'
It was Plato who said, “He, O men, is the wisest, who like Socrates, knows that his wisdom is in truth worth nothing”