High RPMs shouldn't be a problem, assuming leg extension is dialed in and one is staying in control. They can be good for clearing lactic acid after hard efforts.

If anything, I feel like riding fixed-gear with a reasonable gearing has made me stronger overall and taught me some good lessons for knee health that can carry over into other riding. Namely, that you can get a lot more speed out of a particular gear than you might imagine (and spinning up my current gear a little rather than trying to stay on top of the biggest possible gear keeps me fresher longer), and that if you're on a FG/SS (or in the lowest gear on your derailleur bike) and you need more grunt to climb a hill, it's way better to pop out of the saddle than to stay seated and grind your way up. I might get away with the latter approach a couple times on a ride, but I'll feel it later, and it can't be good for me in the long run.