Why do you think so?
Kasparov is slaughtering Nakamura and So.

No he didn't cheat, blitz rule. He had to move the knight but could still put it to another square becaause he hadn't pushed his clock.

If this were a FIDE rated blitz tournament, that move would stand. I assume they're playing clock move though.

where to watch?
https://chess24.com/en/watch/live-tournaments/ultimate-blitz-challenge-garry-kasparov/4/1/2
Kasparov cheated vs Nakamura.