This is definitely untrue and the reason some games have 18,000 mods and some games have 0 is almost entirely down to developer cooperation.

Sometimes if a game is using an existing engine that is known to be moddable, you can get a community built off of some pre-existing knowledge and kind of strike out on your own to build a mod. In most cases if the devs didn't build the game with mod support in mind you're not getting any mods.