I see it quite differently.
As a developer you have access to the source code but that doesn’t make the engine moddable in itself
true, but the engine doesn’t make it easy. unless things have changed recently the engine doesn’t help with things like file discovery (i.e. extracting a custom made package somewhere and having the game be aware of it), “understanding” and integration of such external files into the game, actual creation of mod packages, etc. as it is right now it’s not only hard for us at the developer side to make a game moddable, it’s also hard for the end users on the modding side (they pretty much have to become developers themselves)
pretty sure that modifying the source of a game and re-compiling your own game exe doesn’t really qualify as a moddable game. and I hope you’re not implying that making each our games open-source is the proper way to have modding 