That's not David's apartment... That's V's penthouse apartment from the "Path of Glory" ending of the base game. And trust me, there's a much easier way to get to it than the path you took. It starts inside the Megabuilding H10 where V's original apartment is. People have been exploring this penthouse since the game first came out. If you get inside using clipping mods, you can find a special set of V's clothing... You managed to get inside a dummy version hidden behind the proper version. If you look through the front entrance when you were outside, it was fully furnished, and If you noclip through the door, you can explore the fully furnished house including some shards, guns, stash room and aforementioned V's clothes... But yeah, it most definitely is not David's apartment.