Nomad
Master Black Belt
There is a big difference.
If I've mastered the double spinning jump kick (or whatever) but age catches up with me, I still undertand it because I know it, even if I can't execute it anymore. Therefore I will be able to teach it by instructing the students and tell them where to adjust, based on the fact that I know what they are doing and why it isn't working.
If I've never been able to master it even remotely, then I really have no clue what I am doing, and I cannot teach it to my students since I don't know what to say to them or explain what they need to do.
Ability may fade, but the understanding will stay. If there was no ability to begin with, then there will never be an understanding either.
This makes the assumption that in order to understand something, you have to be able to perform it. While it likely helps, I don't think that's a prerequisite. If someone can teach another (likely younger, more athletic) person how to do a technique properly, I think that in itself demonstrates a solid understanding of the technique, even if they can't do it themselves.
It goes back to the concept that the best performers aren't always the best teachers, and vice versa.