Guam (PGUM) is not a military airfield, that is Andersen AFB about 10 nm to the SE. Guam is a civil airport, nothing else. I think there is also a Navy airfield to the west but am ready to be corrected on that.
Radar services there are excellent, from experience.
As for it being an "unknown airport", I would have expected the crew to have been thoroughly briefed seeing it was on their route and most likely an EDTO alternate. Regs require crew on RPT flights to be so briefed. Maybe they hadn't ever operated into the place (although if the airline is using the sim properly the crew should have been there in that manner, at least) but they should certainly should have had a working knowledge of it. If they hadn't then something is amiss.
Not taking away from their response to the situation one iota, just let's keep it factual.