Yes, you're right it seems.
Is there any way to keep the initial surface mesh? I assumed tetgen wouldn't
change the surface and would just fill the volume based off of the surface.
The volume constraint I was hoping would allow me to create consistent sized
tetrahedrals by calculating the tetrahedral area using the average face edge
lengths of the surface.
No, not as far as I'm aware. In Triangle (the 2D mesher in meshpy), you
can allow or forbid boundary Steiner points, but I'm not sure that
tetgen allows the same. TetGen's documentation is really the right place
to look here; or maybe you might be able to ask Hang Si.
Also please take care to keep the list cc'd on your replies.