It may be open source, but it's not open source as I've ever encountered it before.
(Also, I don't think I've ever heard someone claim that copyright law allows them to demand royalties on the use of a musical instrument. Is a musical performance a derivative work of the instrument design?)
Releasing just .STLs is more akin to freeware than open source, as the sources are the project files uses to produce the .STL (which is similar to a compiled binary).
STL, on the other hand, is a text file that can be imported for most 3D design tools and hence you can modify it.
Say for instance you want to do something simple, like use another sized tuning pegs for this violin. That means changing the diameter (and possibly depth) of the holes they go into. In any reasonable parametric CAD software model (as Fusion 360 enourages), this is merely finding the sub-part that represents the peg or peghole, and entering in a new value for the diameter - every affected area of the model will be updated, correct & still manifold (watertight). With an .STL you will need to find & select all the relevant edges/vertices for each hole, and nudge them around until they (hopefully) fit. This is a trivial example, it becomes more interesting (more painful) if the feature at hand is more complex. Say if there was a trapped nut.
In an STL, all the humanly meaningful parts of why the model is the way it is, how it is built, is gone.
You can dissassemble some executable you get too, and muck around with the code to do modifications - but its hardly how one should do open source.
A .OBJ file would be the same. I'm not entirely sure what others sources may be useful.
There's a repo on GitHub, but it looks old https://github.com/matthova/hovalin