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.

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.

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.

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, and even that resemblance is unlikely to be noticed by anyone just thinking about the issue, and is rather spread as a kind of a perverse meme.

Methinks thou dost protest too much.

The earliest time I can remember that anyone drew a very explicit connection between simulations and theism is in Stanislaw Lem's short story about Professor Corcoran. The book was originally published in 1971, when Bostrom was -2 years old. It's in the second volume of his Star Diaries; see "Further Reminiscences of Ijon Tichy: I" in this (probably pirated) scribd doc. I'd recommend it to anyone. Of course, it's very much possible that Lem wasn't the first to write up the idea.