How many stuttering acting presidents do you remember?
Also Biden isn't just having a stutter. It was clear he lost his train of thought multiple times, like destroying medicaid, constantly mixing thousands, millions, billions and trillions or his talk about abortion trimesters.
This sounded like they gave him elaborate sentences to remember beforehand and he butchered them all, because of not learning them by heart and not fully grasping what they actually mean.
Meanwhile Trump was just doing Trump and Biden came wholly unprepared for that. Repeating what Trump said with "the idea that ... is ridiculous" is just repeating what he said, further giving it agency. How could they be unprepared for Trump being Trump after 8 years of him already?