I think the example diagram is incorrect. The Fox original diagram is :
White : Ka3 Pf5 Ae8g1
Black : Kd5 Rb2d1 Nb8a5 Pa7a6f6c4g4c3g2 Ah5
There is a twin with b) Pa7->é2
The solution is
a) 1.c2 Ab5 2.c3 Axg2#
b) 1.g3 Axb8 2.Af3 Axf3#
Both model mates.