In this puzzle both Nxf7 and h6 result in mate in 6 according to houdini. The Lichess puzzle only accepts h6 as correct though, and counts Nxf7 as incorrect. An example of a Nxf7 mate in 6:
1.Nxf7 Re7 2.Bxe7 Qf1+ 3.Kxf1 Kxf7 4.Qe6+ Kg7 5.Qxg6+ Kh8 6.Qh7#
Here is the training puzzle: lichess.org/training/39120
1.Nxf7 Re7 2.Bxe7 Qf1+ 3.Kxf1 Kxf7 4.Qe6+ Kg7 5.Qxg6+ Kh8 6.Qh7#
Here is the training puzzle: lichess.org/training/39120