Why was it not checkmate?
Because if kh1, diagonal queen check. Can't go down obviously to kh3 or kg3, nor kg2 or kg1. The king legally cannot move to any surrounding squares because they are covered.
The guy might have figured it out in the last 7 years. Maybe by reading a dozen comments from 2019 that said the same thing.
It's a stalemate, because if the white king doesn't have any moves it's a draw not a lose. That's another rule in chess 😄
It's stale mate... the king is not in check and can't move anywhere, nor does he have any other pieces to move
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