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Hrqls: Article 3.9 of the Fide rules of chess say:
The king is said to be 'in check' if it is attacked by one or more of the opponent's pieces, even if such pieces are constrained from moving to that square because they would then leave or place their own king in check. No piece can be moved that will either expose the king of the same colour to check or leave that king in check.
Your movement of the knight would leave your king in check, contravening the rules of chess. Hence I believe Brainking is applying the rules of chess and hence three check chess correctly.