Although I agree with you mrmip, that does not seem to be the case. I have seen a few chess puzzle books that have en passant puzzles, but do not tell you this was the last move.

Although I personally find them annoying, they seem to be quite valid in the world of chess puzzles.

Technically a puzzle with castling is bad too, since you can't prove the rook and king haven't moved.

So I do think this puzzle is quite valid, one example of the castling and en passant puzzles appear in Polgar's book of 5000something puzzles, combinations, etc.