I agree with you about it possibly being dementia. I also work professionally with older adults, many with dementia, and what you describe could fit. The issue of continued competence as clinical professionals age is a tough one. You might want to consider reporting this incident to the licensing board. They can take it from there, but at least you would have provided a data point that may or may not turn out to be something requiring action.

Aside from that, it sounds like a very distressing interaction. Good for you for staying grounded and responding calmly and logically. That's really hard to do when someone is railing at you unfairly.

gg