Let me preface this by saying clearly - I AM NOT A DOCTOR - but, I believe Alzheimers does begin to cause damage to the brain years, even decades, before any of that damage is evident in the patient's cognitive abilities.
So, physiologically, it's possible and probably even likely that Reagan's brain was beginning to be effected by the disease while he was in office, but that's a LONG WAY from saying that his cognitive abilities were in anyway compromised by the disease during his presidential tenure.
That's the little tutu-wearing weasle's only loophole that cannot be proven one way or another. However, it's not the first time the Nancy-boy has slandered his father's legacy. He should change his name to Alec Baldwin Jr.