I usually support the police.
It was pretty clear from the way she was acting that she had significant cognitive impairment.
Wandering dementia patients is not an uncommon issue that the police have to deal with.
Being in the moment, when your life is constantly on the line, very little is "pretty clear" about someone's medical condition aside from obvious external trauma. Yeah, that person has a missing arm. Pretty easy to diagnose that.
She could have been on drugs.
She could have been acting. Some people are very good at faking things. Again, difficult to 100% definitively say on the scene.
LEO encounters every walk of life and then some. Sometimes they believe a person is one thing, and it turns out they are something else in a matter of minutes (or even seconds).
Law enforcement isn't in a position to diagnose people and their condition. They don't have the training, and almost always don't have the time.
People should not expect law enforcement to also be clinicians who can diagnose someone's condition.
That being said, the way in which they treated her...regardless of what her condition is or isn't was reprehensible. What was clear from near the beginning was that she wasn't a threat to them and the sinister mocking afterwards is twisted.