They make the game with these tools, as in they put everything together, make the world, quests, NPCs, scripts etc.
But the engine isn't made in the Creation Kit. Neither are the meshes, or the textures, or the shaders. These are all used in the Creation Kit, as in loaded in and used, but they're not made in it. In Morrowind, Oblivion and Fallout, spell effects and skills were hard coded into the engine. You could not change them in the Construction Set/GECK, because they were simply added on an engine level by the programmers. If an employee needed a new spell effect, they'd have to ask the programmers to code it in for them, this has been said by an actual Oblivion developer. We obviously can't do that.
So for all intents and purposes, things like spell effects and skills are hard coded into the engine. We cannot change them or add new ones.


