Not sure what evidence you've been looking at; from what I've read the opposite is true: Keto diet can be an effective treatment for Alzheimer's.
https://pubmed.ncbi.nlm.nih.gov/29701155/
https://www.nature.com/articles/s41598-020-65908-y
But can't say I've been following Youtube videos or Reddit on the matter.
Edit: I didn't know who the youtube video you linked is, but a cursory google bring up that he is accused of quackery. Also, the source on that video is... his own website hosting his own work. Not exactly peer review is it?
You can check my other comment above for a bunch of other sources/studies on keto diets reversing Alzheimer's symptoms if you'd like.