I mean... I really can't say with any certainty, but at this point, it would be fair to speculate that there is some of that going on. Now, I don't think it's players actively not giving their all--it's more of a morale issue. It's distraction and fatigue and could be uncertainty out on the field due to a lack of answers coming from the top.
All of that makes a difference when it comes to a step here, an yard there--the stuff that separates making a tackle from missing it. Delays in diagnosing the play because you're getting guys lined up late. Freelancing a little more or getting outside of your assignment b/c you don't trust that the guy next to you is sound.
So I think "quit" is the wrong word. I think guys aren't playing as fast as they could be b/c the entire defense is mired in uncertainty and maybe a lack of assignment clarity--and, perhaps even, just too many players not buying into the defensive philosophy. When something isn't working, as a coach you're on the clock to figure out how to make it work before guys stop believing in it. And when you're out on the field, having to run a concept you don't believe in, that, in itself, is a distraction that just adds to the cascade.