I feel like I've seen enough of these "X is good/bad for Y" articles on these popsci explainer sites surrounded by the same ads for an entire lifetime. Not that I don't think this is actual research, it's just the canonical link we end up with here on HN is probably... not the ideal one.
I wanted to grab the journal link, but going back to the article now, I'm trapped in a "keep reading" loop that blocks the article, so I can be told "Mouse Study Suggests Nose-Picking Has a Surprising Link With Alzheimer's". If I could use an adblocker on the work PC I would.
It's not directly obvious that sitting in a certain way activates the brain more (it probably does if any balance center is activated), but sitting in better posture does have a lot of benefits from breathing better, not having muscles worn in overdrive to try to compensate and so on.
Playing chess in one leg is a curious thing to me, I think it would initially affect the player's performance but that's just a guess.
Next: Playing chess on one leg is better for brain health than sitting.