This seems weird to me.

It seemed weird enough to me that it stuck in my memory more clearly than any of his anti-MIRI comments.

XiXiDu does not strike me as someone who is of average or below-average intelligence--quite the opposite, in fact.

I concur.

Is there some advantage to be gained from saying that kind of thing that I'm just not seeing here?

My best guess is an ethical compulsion towards sincere expression of reality as he perceives it. For what it is worth that sincerity did influence my evaluation of his behaviour and personality. XiXiDu doesn't seem like a troll, even when he does things that trolls also would do. My impression is that I would like him if I knew him in person.