Why is this move brilliant? I don't see it.

It's brilliant because if the queen takes the rook, move the c2 pawn to c3 to attack the queen. The best place for the queen to go now is probably to go to e5 now offering a queen trade (what I would do as black assuming I made the first blunder) Now I would move pawn to f 4 attacking the queen once again. FYI any other queen move would either get the queen killed or blunder mate in 1. The black queen now really has nowhere to go so at best they lose their queen or if they blunder even more it's a mate in one. If you don't see it once the queen is off the long green diagonal, queen to g7 checkmate.


After taking the queen, the knight checks the king blocking the black queen's protection of g7 where mate follows..
It's 18. Rxd4, suggesting that (like what you noted) there was a bishop on f4 (or it could even be a promoted queen, who knows).