This is taking the term way too literally. He's undisputably the protagonist of both movies. The plots of both revolve around him, the team converge around his actions, including rallying them into "giving a shit" at the end. In terms of the symbolism of power that the interview is talking about, Peter Quill is the leader of the Guardians. That's why the poster, which that panel is emulating, places him most prominently in the center.