Biden is so used to the earbud, he just repeats whatever he hears.
That, possibly, and the fact that it is common for demented people to do what is called mirror speaking— repeating the last phrase immediately heard that they can duplicate and try to cover that they do not have in their mind a thought out response— because there is no longer one there.
Mimicry— to pretend to be part of the conversation and maintain facade of cognition. Stroke patients do this too- as a coping mechanism to “buy time” until or ... “if” they can have a cogent reply in their mind.
This exchange is seriously diagnostic. Goofy JoeBama.
Very possible. If true, we can look for more instances of this in the future.