Pillars of Eternity was moddable only because it was written in C# (Unity iirc), making it possible (though not straightforward) to more or less modify the code however you want. So a mod was essentially a bunch of overwritten classes.

A game that you could literally just fork on GitHub and change things would certainly be moddable in a sense… Handling multiple mods without conflicts, however, is another thing. For that you need a proper modding interface, like the original Baldur’s Gate or Skyrim.