Pure, unadulterated BS. I was in the hospital for a few days last year (unrelated to Covid) and as part of the standard discharge process, I had to walk around (with a mask on) while my blood oxygen was measured. It was slightly low, so the nurse said "take off your mask for a second" and sure enough, it popped right back up to normal. She said she sees it with almost everyone.

You're breathing in less air, and more CO2. There is no way it's not going to have an impact.