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.

I think you're wrong on similarity [1] and irrelevant on ancestry/inheritance. Only some among currently active religions are clearly "related" in the sense you employ (e.g. Judaism and Christianity); there's no strong evidence that most or all are so related. Since you presumably have no problem lumping them together under "religion", the claim that BTanism (grouped and named so purely for convenience) has no common ancestry with these religions is irrelevant to whether it should be judged a religion.

Also, I don't read the post as claiming "you guys are so dismissive of religion, but you're big on BTanism which is just as much a religion, so there!". Instead, I read the post as claiming "you guys are unreasonable in your overt dismissal of theism and your forceful insistence on it being a closed question, considering many of you are big on BTanism which has similar epistemological status to some varieties of theism". So it doesn't matter much whether BTanism is a religion or not; if that bothers you too much, just employ Taboo and talk about something like "a sentient being responsible for the creation of the observable universe" instead.

I don't fully agree with this idea (the post's argument as I read it), but I find myself somewhat sympathetic to it. It is indeed true in my opinion that the overt and insistent dismissal of theism on LW is a community-cohesiveness driven phenomenon. There's illuminating prior discussion at The uniquely awful example of theism.

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

No, I have no doubt that you believe what you're writing. Rather, I think that the strongly dismissive claims in your first comment in the thread, unbacked by any convincing argument or evidence, cause me to think that a strong cognitive bias is at work.

[1] Really, the similarity is so strong that I see no need for a detailed argument; but if one is desired, I think Lem's story, to which I linked earlier, serves admirably as one.