True. BBS has only been out for five or six months (I forgot when it came out in Japan.) and the remnants of one world was already found, though there might be more. I heard that they also found remnants of a Sword in the Stone world and a placeholder name.
Which I think was either meant to be the original name for Keyblade Graveyard or the rest of Mysterious Tower's world, not its own world. Perhaps, they intended on there being more to Mysterious Tower, but they scrapped it. My reasoning for why it was separate from the KG's file is that it was the original name for Keyblade Graveyard, but when they scrapped the name (in favor of KG), they made a new file and forgot to delete Mysterious Wasteland. In Mysterious Tower's case, perhaps they were building somewhat separately and intended on either merging the two files or linking, much like Twilight Town was linked to Yen Sid's (Mysterious) Tower, but ended up scrapping the idea.