I'm a bit sceptical on this. It makes sense as a statement, but it's not like non-fell running old people have perfect knees.
I think if I were to sum up the available evidence it would be something like:
1. People's knees deteriorate as they get older
2. We don't yet know if running (or fell running) speeds this up or slows it down
I remember what John Wild said in that book about him and Kenny Stuart. Something like: plumbers and electricians have bad knees.