“Just do it” is what neurotypicals say when they want to give advice. “Just exercise more”, “just do yoga”, “just make a list”, “just take out the trash everyday”, “just put it away after you use it”, “just fix your sleep”, “just make an effort”, “just be normal”. It’s not advice, it’s unhelpful and talking down to a child.

Honestly weird that you say you haven’t encountered it.