It is not checkmate if the other player has any legal move that gets them out of check. Capturing the checking piece is one such way; whether the capture is en passant or not is irrelevant for the purpose of this question.
In this case, the en passant capture is the only legal move. If the other player didn't know about it, I suppose they might lose on time unless they figure out what's going on first. Using a website such as chess.com one might be able to discover the rule accidentally, since clicking on a piece normally highlights all the possible moves for that piece.