Fandom literally has a strict rule against all words or phrases that are derogatory by nature, let's not forget that this is still the same platform where people have been globally blocked across fandom just for saying the N word or R word even in context that was used for educational purposes not intended to be bigoted. Which even I think that's extreme practice, but they still own the platform and therefore is a TOS who have to follow.
Furthermore, we have always had a site rule that forbids all types of derogatory comments, not just ones certain political parties only focused on a few selective groups, more info
explained here. The definition of Ableism is any type of derogatory or bigoted word or comment spoken against any type of disability whether physical or mental. And Alzheimer's disease is considered a mental disability/illness thus discriminating or mocking it would fall under ableism territory. As for why it's offensive, a person who actually has that could be searching through the internet or the forum and reading that out loud and either taking offense or hoping whoever said that gets brought to justice.
People come here to get away from political drama or from their discrimination heavy environments they most likely face in the real world, which is strictly the reason why there are even rules against making such comments in the first place. It doesn't matter which political party or religion you come from or what your family has primarily been, rules are still rules. We don't have ill will against anything like that, but making the community a derogatory free environment with no hate speech.