I’ve gotta say, after reading his tweet I was struck by how strangely he phrased it. Most people would say something generic like “People are claiming I had a stroke and that’s not true”, but he said such a specific thing and used the phrase “…having suffered a series of mini-strokes…” which is the kind of word usage someone would use for a factual or historical statement, as opposed to the normal generic handwaving that a regular person would do when they want to discredit something. Suffered? I mean, it’s a technically correct word, but not for someone who’s rebutting a lie. It’s such a weird way of saying it.
By “a series of mini-strokes” he is likely referring to vascular dementia. Its a progressive, step-wise decline in mental capacity accompanied with neurological deficits due to multiple, minor but accumulating ischemic injuries to the brain. It’s often described to patients as “mini-strokes” in layman terms. This is such a specialized topic that it is unlikely he would know about it unless someone, in this case likely a doctor, specifically described it to him in such terms. And it’s hard to think of a reason why someone would go out of their way to describe this diagnosis if it wasn’t suspected.