They are employed by themselves. They are not employed AS anything else. You have it right just your conclusion is inconsistent.

It’s for me not about the wording of the last paragraph by the way but about the context and requirements list which makes the impression (to me) of offering an employee/employer relationship which is only broken up in the last line. That’s the part I really don’t like.

This kind of advertisement would be illegal in Germany btw as it would encourage pseudo self employment: someone self employed who is relying on one client only. (And no, not exaggerated: I’ve a legal department at least pull job description from the tech dept similar to this).