It looks like I need a login to get the article so I haven't even read it. So I have no evidence, articles or anything, but I think there must be a difference between entertaining oneself during life's inevetiable down times and what you refer to as behavorial disengagement.
I see what gratified's child is doing is much more in lines with self-comforting and self-control (which are positive traits)rather than checking-out.
I personally just don't assume it's harmless, whatever you call it.
That's not to say that I assume it's harmful either, BTW. Just that I think the question is important to ask in the generic sense. (None of this is really aimed at gratified. As I said, I never intended to second-guess. I would never presume.)
It's possible this sort of coping strategy is positive but the situation that requires the coping strategy is not, at least not in the long term. I think that would be my take on it, FWIW.
Important, too, is the fact that my parents and teachers always assumed it was a positive thing that I stayed busy and undisruptive, I think. A big part of this is my own rejection of that assumption.
