Astounding that swimming and collisions and all that stuff seems to function!

Commenter explains how it works:

  I can see not enough people know how this addon works. Allow me to explain.

  It is a mod, but it was only possible with a plugin, it can be (with a bit of  tweaking) used for other games and already has been, for example rocket league, sonic generations, blender and minecraft. the way it works is that mario 64 is  running in the background so that the mario physics and gameplay functions work, but the actual character is invisible but the hitbox is still there, so that mario actually can be hit for example. with the plugin mario can pretty easily be put into any game, though some fixes might be needed at some times, for example in sonic generations for the rails to work, mario has to be disabled and sonic's control is taken over again so that "mario" can grind on rails.

  TD;LR: Basicilly this addon use libsm64 which contains original SM64 assets and ported into GMod. So yeah, it's not a recreation and that's also explain why you need a Mario 64 ROM (in z64 file format) to get this addon working.

Eagerly awaiting my new plumber clan in VtM Bloodlines

How many Portal test chambers could be solved with Mario 64 jumping alone, and no use of any portal gun?

That is amazing and it makes me smile