For one, your memory is just all over the place. You wouldn't be able to hold it together long enough to read from a teleprompter.
Think about if you ever had anyone close to you live with the disease. It gets worse day after day, its random when/if they have moments of clarity, they need help doing a lot of stuff (depending on how far advanced it is), and its a lot of repetition, talking in circles.
I am officially on the paid actor bandwagon. Something is just ... off about the whole thing
He's been giving speeches this whole life. Been reading from teleprompters for nearly two decades. That practice likely makes it easier for him.
It's conceivable that they keep him hidden until he has a rare moment of clarity.