Good question!
I was aware of the difference you are alluding to, but forgot to check the bottle. However, some googling just told me the Carib FES has an ABV of 6.5. The stuff I was drinking in St. John was 7.5 ABV, the same as the stuff we're now getting here. So I would guess it was via the mainland. They sold it in 6 packs for $11.99 (I've only seen it in four packs here), so that was actually one of the few "bargains" we encountered on anything in St. John.
They also had various beers by something called St. John Brewing Company. We had a pretty mediocre six pack of IPA (bought it when we first got there until we discovered a better beer store). Turns out it's just stuff shipped in by Shipyard Brewing Company from Maine.
The one store had a handful of Belgians (e.g. Chimay), but at $29/4 pack (for Chimay) (vs. 11.99/6 pack for FES and Celebration Ale), we passed.
Those were my findings as well.
Though I was drinking it last week in St John, USVI, so that helped the taste a bit.
St John was a craft beer wasteland. I drank Sierra products and Guinness Foreign Extra while there.
Did you happen to catch who brewed the Guinness FES? Carib brews it in BVI. Curious if USVI is getting the new supply from St. James via the mainland or the Carib variety. The Carib version has a lower gravity profile that would be more drinkable for the tropics.