But then I see them proceed to self-describe as atheist (instead of omnitheist, theist, deist, having a predictive distribution over states of religious belief, et cetera), and many tend to be overtly dismissive of theism. Is this signalling cultural affiliation, an attempt to communicate a point estimate, or what?

To a non-scientifically-literate person, I might say that I think electrons exist as material objects, whereas to a physicist I would invoke Tegmark's idea that all that exist are mathematical structures.

One way to make sense of this is to think about humanity as a region in mind space, with yourself and your listener as points in that region. The atheist who hasn't heard about Bostrom/Tegmark yet is sitting between you and your listener, and you're just using atheism as a convenient landmark while trying to point your listener in your general direction.

It may be that theism is in the class of hypotheses that we have yet to develop a strong enough practice of rationality to handle, even if the hypothesis has non-negligible probability given our best understanding of the evidence.

Why do you say that? I don't think anyone has gone mad or otherwise suffered really bad consequences from thinking about Bostrom/Tegmark-like ideas... (Umm, I guess some people had nightmares after hearing about Roko's idea, but still, it doesn't seem that bad overall.)