Lesson 4.14 (removing the guard) 9:21, I think I found a mistake there. You say Qf8 leads to mate on next move, that is correct. But black has the option to instead do Qe8 and it would “only” lead to a skewer after Rxe8 to win the second rook. So the sequence is still correct, but it isn’t a forced mate, just a queen and a bishop for a queen, a knight and two rooks and of course a very winning position.