I had given up on this fare, but chose to take another look at it tonight, and I have finally figured out what's been making me crazy about!
Apparently, even though this this is technically a flight to Guam, you cannot stopover on Guam. You must depart within 24 hours.
Please someone correct me, because I feel like I've been eating pot brownies or something.
Another thing is, even though the Island Hopper flights on Sunday trigger the fare, you can also take the non-stop GUM-HNL, which, uhm, might be better for me. Haven't thought that far in advance.
But if I can't stopover in GUM, it's moot to me, as that's where I want to go.