It is explained, although in very general terms. Exercise is used to stimulate the body’s “countermeasures” (my word) against neuro-degeneration. Squats are called out here for a few reasons: They’re relatively easy for most people to do, and they seem to do a good job of triggering the “countermeasures”, as best as they can tell.
This is not a prescription to drop your exercise routine and only do squats, it’s really just pointing out that squats are a good first choice for sedentary people who want to keep their brains healthy.
There were a few people in the video squatting with their knees way too forward rather than squatting like they're sitting down. It's a pretty common beginner mistake. I agree that they're easy enough--it's not like they're suggesting the snatch.
In all the squat videos I've watched, knees go forward almost to the maximum that the ankles can sustain. For example, Jeff Nippard's tutorial: https://www.youtube.com/watch?v=bEv6CCg2BC8
This is not a prescription to drop your exercise routine and only do squats, it’s really just pointing out that squats are a good first choice for sedentary people who want to keep their brains healthy.