Have you tried just converting the files in solidworks? I think he uses solidworks to create them.
If you get an older version you can use the Autodesk Mesh Enabler, an add in for inventor, to convert the stls to a base feature. It can even give you an axis through a hole to constrain to. Inventor Pro 2017 actually handles stl files really well in assemblies. You could download it and do the assembly and then be done with it (or activate if you are a student).
Autodesk offers the full Inventor Professional to students. However, you have some options to use the stl files on thingiverse. Understandably, vicious has not released any of the actual cad files besides a dxf of the motor mounts and maybe a beta part here or there.
