I hate how this situation and explanation kinda make sense, but not, at the same time.
The body does adapt to circumstances it's put in, but that's really not how strength works. The usual saying "if you don't use it, you lose it" is exactly how it works, but within the paradigm of this story they can't actually lose their so-called skills. So next thing you'd postulate is what is said here, the body loses muscle but it'd be circumvented by being stronger. Somehow.
It's one of those.
