I once went to a presentation by a Black doctor who talked about a time when she had newly qualified and was working in a hospital in North Wales, which is an extremely White area. There was only one other Black doctor in the hospital, an older man. Many patients kept confusing them for one another.
How do you keep confusing a younger woman for an older man? Well, if you're racist enough then you only notice the skin colour and everything else falls away.
This is what I think is happening here. The fact that she's elderly and has Alzheimers doesn't matter to a racist: all that matters is that she's Brown. Once they see that, their racism prevents them from noticing that she isn't an athletic military-age Hamas gunman.