If someone told you 10 years ago that they were going to create something where you could play a whole new level of Doom, without them writing a single line of game logic/rendering code, would you say that that is simpler than creating a demo by writing the game themselves?
There are two things at play here: the complexity of the underlying mechanism, and the complexity of detailed creation. This is obviously a complicated mechanism, but in another sense it's a trivial result compared to actually reproducing the game itself in its original intended state.
