I believe you are right, Angore.
Further, a good number of the 3,000 population are US service personnel who use US stamps to send their cards and letters back home from Diego Garcia. So bona fide non-philatelic use of BIOT from Diego Garcia or any other of the Chagos Islands must be pretty unusual.
Just curious to see, I did a quick and not-at-all extensive search of eBay
and see two non-FDC uses right now that might really be non-philatelic. Both are going to a Harry Smuckler in Philadelphia PA. Here's an image of one such cover, followed by the link to the listing (Note: not my listing - seller is in Argentina of all places for such a cover to wind up.) https://www.ebay.com/itm/255107202812