The context is very important: decades of a poorly-diagnosed chronic illness had left him deeply distrustful of the medical system.

This is the real root issue.

At 75 years old, he was stubborn. Is that reasonable ? Yes, perfectly. Could he have been right since the beginning ? Certainly. Did he deny evidence ? Yes.

Zero doubt that he was intelligent, everything points toward that direction, but that doesn't make a person less stubborn, because accepting the evidence, is also accepting that you were wrong if you initially postured yourself as adversarial instead of cooperative.

He would have read Wikipedia, scientific papers, etc, even without AI.

He did not want to be convinced. It works both ways:

https://www.foxnews.com/health/woman-says-chatgpt-saved-her-...

or

https://www.today.com/health/mom-chatgpt-diagnosis-pain-rcna...

Nonetheless, someone very smart, just didn't want to move from his position.