If memory serves, they're actually in a fixed position, so yes, they hover. At least, back when I had an anchored fleet carrier. Could even call it a geostationary orbit, but personally, I've never measured if the carrier really does orbit at the necessary height. I don't know why Frontier decided to make them this way.
However, they might have implemented it by making carriers actually a surface base, just set to a height level that's far above the actual surface. That would explain why the gravity at surface level would prevent players from disembarking in orbit.
On the other hand, I could easily be misremembering and thus be wrong, and the game just mistakenly checks the surface gravity for disembarking.