But it didn't sound like Pauln6 was talking about a diegetic take on the Many Worlds Theory, but rather applying it from an external perspective.
Yeah, but still in the context of a conversation about a fictional universe, in which it's assumed that timelines split as a result of human decisions and macroscopic outcomes. If that's the way multiversal physics is presumed to work within the fictional universe (and it almost always is), then it's valid to talk about the fictional universe in those terms, even though it's different from real physics.