I think you're describing the common interests of the tribe, but that's a different thing than the rallying flag.

Since we're operating within Yvain's framework, we'll use his definition which is

The rallying flag is the explicit purpose of the tribe. It’s usually a belief, event, or activity that get people with that specific pre-existing difference together and excited.

HPMoR, for example, is (was?) a rallying flag for a subset of the LW tribe. But I don't think a "default disposition" would qualify (Yvain would call it a stage 1 "pre-existing difference") and an "assumption" is very doubtful as well.