Isn't this - I'm sorry if that sounds harsh - arguing by a forceful say-so? Sure, if you constrain theism rhetorically to "Jehovah-worship", that practice doesn't sound very similar to the Bostromian arguments. But "Bostromian arguments/Tegmarkian speculations" and "the claim that a god created the universe" sound pretty much memetically related to me.

See Religion's Claim to be Non-Disprovable for discussion of what religion is and how it arose. By "memetically related" I do not mean "memetically similar" (although I don't think there's much similarity either); I mean "related" in the sense of ancestry/inheritance. Bostrom's and Tegmark's arguments are not a branch of religion; they do not belong in that cluster.

You're saying that e.g. "we are living in a simulation run by sentient beings" and "we are living in a universe created by a sentient being" are such wildly different ideas that there's only superficial resemblance between them,

No. The implication of the post, as I perceived it (have a look at its first paragraph) was "you guys shouldn't be so confident in your dismissal-of-religion ('atheism'); after all, you (perhaps rightly) are willing to entertain the ideas of Tegmark!"

Surely you understand what is wrong with this.

Methinks thou dost protest too much.

You think I don't believe what I'm writing?