If I had said, "I've lived in enough places to conclude that southern California is really the best place" would you read that as me making a universal claim or simply relaying my subjective determination?

As I already stated, I read the OP as if the latter. Despite the often pedantic nature of HN, there's a lot of room for nuance and interpretation human communication.

That depends. If you had prefaced your remarks in the following way, then I would read it as making a universal claim.

"Bluntly, every person I know who's expressed these kinds of sentiments is guilty of the same mistake: [living outside southern California]."