Sorry, he really should have written "gcc, clang, and EDG" monoculture.
Are you aware of any significant compilers that use anything other than these as frontends at this point?
(microsoft, intel, portland group, comeau, etc all use the EDG C++ frontend)
I guess XLC still does (or it did when I was at IBM 7.5 years ago), but not sure they are really still in the running.