Many folk here on LW take the simulation argument (in its more general forms) seriously. Many others take Singularitarianism1 seriously. Still others take Tegmark cosmology (and related big universe hypotheses) seriously. 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.

The word "but" in the last sentence is a non-sequitur if there ever were one. Tegmark cosmology is not theism. Theism means Jehovah (etc). Yes, there are people who deny this, but those people are just trying to spread confusion in the hope of preventing unpleasant social conflicts. There is no legitimate sense in which Bostromian simulation arguments or Tegmarkian cosmological speculations could be said to be even vaguely memetically related to Jehovah-worship.

The plausibility of simulations or multiverses might be an open question, but the existence of Jehovah isn't. There's a big, giant, huge difference. If we think Tegmark may be correct, then we can just say "I think Tegmark may be correct". There is no need to pay any lip-service to ancient mistakes whose superficial resemblance to Tegmark (etc) is so slight that you would never notice it unless you were motivated to do so, or heard it from someone who was.