You shouldn't assume the suspected Blue's observation is a pro-moon shot or anti-Green argument.

("Shouldn't assume", taken literally, sounds like an endorsement of forming beliefs for reasons other than their correctness. I think I agree with the intended point, but I'd put it somewhat differently.)

Rather than focusing on the factual question of whether a remark is motivated by identity signaling, it's sufficient to disapprove of participation in any moves that are clearly motivated by signaling or engage with the question of whether other moves are motivated by signaling (when that's not clear). It's the same principle as with not engaging with attention-seeking trolling: there is no "assuming" that someone isn't acting in bad faith, but engagement in that mode is discouraged.