h8=B is also mate
You obviously didnt look at the move list.
Cnl_Duck: 1...Re5# works, too!
BigDoggProblem: If there has to be a mate, only ...f7-f5 was Black's last move.
But yes, your are right if this were a problem the solutions "en passant" and castling must be proven by retro-analysis. That means the only possibility must be "en passant" is forced possible or it's cristal clear that castling is still allowed.
Cogwheel Post #49: 1 Qf6# works, too.
doduobird123 Post #104: 1...Qg5# does it, too.
Abhishek2 Post #117: 1...Qg3# does it! 1...Qg2 is certainly wrong!
doduobird123 Post #160: 1 Qf5# second solution!
Who is keeping count?
I don't know,
but you can see the post's number