Bug Animation Rigging: humanoid rig is throwing the curled up error shape

I'm following this tutorial:

I'm using a different humanoid character, and 100% won't work with the mechanim set to Humanoid. It throws the no animation curled up error at runtime, only possible fix is to assign an animation but I'm trying to make a VR IK/procedural rig so I don't want it animating on top of the IK.

As soon as I change the mechanim's rig to Generic the animation rigging works as expected. Why is there nothing in the documentation to say "by the way use generic rigs for this"