Well the demonstration uses Skyrim[1] to live-edit the character model which goes beyond the capabilities of a generic mod. Character customizations are also usually premium feature with free-to-play games.

Also, the technical overview describes that the rendering calls are intercepted by matching the "vert/prim count"[2] which is an interesting approach.

[1] https://www.youtube.com/watch?v=HDAN63VyJSY

[2] https://github.com/jmquigs/ModelMod/tree/master/Docs/devguid...