I think the free version is more nuanced then that. By using the free version you agree to:

1.Use it for non commercial projects.

2 Place your designs in the public domain for anyone to use. Any design created from a free account is made public and may be used by others.

I didn’t dig too deep so I’m not sure what restraints are placed on anyone using your design.

But at no point is Onshape claiming ownership of your design.