We won't ever say that it is 100% compatible with any Fanuc. It has similar functionality and most Fanuc 21i stuff works. But Mach 4 can also do things that a Fanuc 21i can't. For example, both G76 lathe threading cycle formats are accepted in Mach 4 where only one is accepted on a 21i. Mach 4's dwell (G04) can be HAAS style or Fanuc style, etc...
So no, the manual will never say this, officially. But a person familiar with running a Fanuc 21i type machine will feel right at home in Mach 4.
Peter Smid has a good book on G code programming. And also a book on Macro B. These two books were used during the development of Mach 4.
Steve