Who is a project manager? I have worked in IT business for 10 years and still I'm not expert in any kind of technical sort of things. The reason is not cause I'm a business owner or a CEO, I've worked as a freelancer, contractor, entrepreneur and a simple employee, and in all cases I was engaged in technical projects without getting too much into details.
I think this is the key to answer who is a project manager. A project manager should be a person with same responsibility to technical and management firms, to coordinate the relations and to transmit the necessities among the parties. In other words it's the pivot to handle the relations between group of managers and engineers.