IIRC neither of those games are examples of uses of the SGB OBJ layer. They're just uses of the SGB CHR layer (custom borders) and of setting sprite palettes distinct from the tile palette (seemingly-custom sprites.) Correct me if I'm wrong, though!

According to https://tcrf.net/Super_Game_Boy#OBJ_TRN_.28Object_Transfer.2..., the SGB OBJ_TRN command wasn't even intended to be used by games! And it was removed in all revisions of the SGB after the first. It's no wonder it went unused. I wonder what Nintendo were thinking, including it in the first place!

But also, to be clear, when I said "fully exploited", the "fully" part is important. :) I didn't mean that nobody has ever used these features in some trivial way in a GB game. (IIRC, the definitive example of a trivial use of the SGB OBJ layer, is in Donkey Kong '94.) I meant that nobody has ever really pushed these features to their limits. There's a lot that can be done with the SGB (https://gbdev.gg8.se/wiki/articles/SGB_Functions) that I've literally never seen a game doing, let alone doing something interesting with.

Using all the SGB features together, you could likely cook up a SNES-looking or GBA-looking game, running purely on the GB's Z80 CPU! Surely not something a time-constrained game studio would care to do; but a hobbyist, as a passion project? Sure, why not?