The key move? b8=black knight! That's the only way that black can't capture. Soon after this hypothetical situation was proposed the rules were changed to only allow for promotion of the same color.
1 e7 kxf3 2 e8=R! Kg2 3 O-O-O-O#!!! (the king moves to e3 and the rook to e2 in a vertical castle. Since the king and the rook were in their original positions, this move was legal)]
Not surprisingly, this rule was changed too.