The question isn't whether an issue is political or not (I'm not sure that's an interesting/meaningful question); the point is to avoid the problems of partisan thinking, and one way of doing that is to pay less attention to political alignment.

If you put a big banner over a discussion saying "HEY THIS IS A POLITICAL DISCUSSION", and you have people adding "AND THEREFORE, REPUBLICANS ARE RIGHT!" at the end of their posts, or reply with "OH, THAT'S A SOCIALIST ARGUMENT YOU'RE MAKING THERE", then everybody is necessarily going to pay more attention to partisan alignment. They may suspect others of trying to advance partisan points. They may be more selective in what arguments they accept. They may be less inclined points that go against their political inclination. It may degenerate into "Well you're just saying that because you're an anarcho-monarchist!".