FALSE. Bethesda is the ONLY one.
I think this is almost accurate.
Though there are quite a few companies that do release modding tools with their games, most are made especially for players to use (ie: Easier to use, less control over actual content, less resource hungry), but there are a few who -like Bethesda- release the same kit (or near enough) that was used to make the game.
I've used a some editors over the years that were so difficult to use and cumbersome for non-programmers that they had to have been the dev tools.
Best example: The map creator for C&C Red Alert was ridicously easy to use. Drag and drop map tiles, place spawn points and features. Save, load game play map.
The creator for Total Annihilation. Still drag and drop, but tiles were all sorts of weird sizes. Different water textures meant finding matching ones was a chore. You had to compile the map when done which took a week for anything that wasn't two screens across and you had to designate voids -places ground vehicles couldn't go on the terrain and if I remember right, set heighmaps as well.