His definition of rare is an artifact of the notation where board state requires disambiguation, as in it includes externalities.
I feel like the question remains unanswered
I feel like the question remains unanswered
So while I agree that the notation is pretty arbitrary and puts lots of emphasis on how implicitly a move can be recorded I don't think it is fundamentally worse than any other definition. Yes, personally I would have picked something more directly tied to the game than if the notation requires more disambiguation, but I don't think it really makes the video any less interesting or the result and less useful (probably no use either way).