He was reading off a prepared script he had in front of him. This would prevent the worst tangents that Alzheimers can inflict on ones train of thoughts.
if you can get access to it,
https://www.j-alz.com/content/track...eimers-disease-diagnosis-case-study-comparing This study is FASCINATING.
In short,m the earliest signs of Alzheimers are incredibly deceptive. Changes in modes of speech, slight dips in the train of thought, that slowly compound over time. What makes Alzheimers particularly interesting is that it has no consistent time of progression. So while he showed signs of it at the start of his second term, his was a slow degradation that had not severely impacted him by the end.