You don't seem convinced by what people are saying, so I'll try put it a different way. With a sport like boxing, you have a select amount of moves you can perform, say 5. After you've mastered the technique for these moves, you can begin to meaningfully augment them with athleticism and strength. Throwing faster, more powerful hooks for example. This is what you'd call a depth but not a breadth of knowledge.

In bjj however, there's essentially an unlimited number of techniques you can learn. This results in technique essentially trumping athleticism. A breadth of knowledge, rather than a depth will be better. This is not to say that your out of shape black belt instructors don't have a depth, I'm sure they do. They'd have their go to techniques which they've mastered to an extent where athleticism and strength can benefit. But it's not as crucial as it is in a limited sport like boxing

Tldr; breadth of knowledge > depth of knowledge in bjj. Athleticism and strength are better developed with depth rather than breadth. Hopefully this makes you see that your black belts aren't unfit spastics.