Never had nor heard of knee issues from spinning.
I’m not just talking about pedalling fast, I mean the sort of spinning where one gets a bit carried away and you almost get a whiplash sort of effect in the knee joint at the bottom of the pedal stroke as the lower leg pushes the thigh back up faster than the leg can handle.
I’m fully prepared to accept that everyone is different both in riding styles and physiology so different pedalling techniques may suit one person over another. I also agree with those who warn against staying seated and pushing large gears up hills whether single speeding or not. This may also affect certain people.
It doesn’t seem to affect me riding geared or single speed but I do that very rarely, preferring to climb out of the saddle when single speeding or selecting a bigger cog when riding gears.