Normally, if a board is reported to work, we get to see a log file including the chip... and if that's not tagged as fully supported either, then we add this too. In this case apparently we did not get a log... but I would assume that it will just work fine. Also, this is not really a warning... we expect untested chips to work in general, but without such we would never get reports on them ;)
Ahh, well, it tends to scare one off from trying. However, with your reassurance I tried anyway and can provide a second confirmation that this motherboard (P5GC-MX/1333) works with flashrom v0.9.7-r1850
Cheers!