His "Calvinism" thing looks completely baseless and arbitrary to me, though, especially in the face of Nisbet's argument. Could it be more of an attempt to sweeten the pill for the "conservative" part of the audience by avoiding blaming "mainline" Christianity?

You are plain wrong on this. I find this suspicious and strange since you didn't used to be.

He explicitly states that American progressivism is the descendant of mainline protestantism. As to his audience if anything most of his "conservative" non-atheist readers are probably protestant and nearly everyone reads him as blaming at the very least mainline protestantism too if not Christanity as a whole. Moldbug does rant less on Catholicism but I think that is because he sees the same thing Muflax speculated on:

There is one idea though that I’ve been thinking about recently. I wondered, what exactly makes the Catholic Church not progressive, in the Moldbugian sense? It has been argued that Christianity is progressivism (and vice versa), and that seems really plausible to me. It’s fundamentally a monist, universalist, transgressive salvation movement.1

Then I got this idea. (And I feel really stupid for only getting it now, when I’ve personally argued every single component of it before.) Catholicism is a containment procedure. The point of the Catholic faith is to defeat Christianity. It’s a long troll.

The first thing Catholics did was to pwn every single Christian movement until only they were left. Marcion got censored, bowdlerized and just plain trolled. Gnostics, Jews and Cynics were absorbed, itinerant and charismatic preachers were shut down, prophecy was officially forbidden.

Then the real work began. They imported as many proven institutions as they could and prepared Europe for the Fall of Rome. (Thanks to which European civilization exists today.) Theologically, they completely neutered Jesus. There is no apocalypse, no call to perfection, no immediate salvation, no suffering to overcome, no secret teaching, no hidden God. And the best thing: Catholics inserted fundamental otherness as a good thing into the teaching. That’s the best anti-progressive troll of all!

This massive undertaking was successful at containing Christianity for a long time. It wasn’t until those dirty Protestants realized that the Church has no intention whatsoever to take itself seriously. They didn’t realize that Christ is a basilisk, and there’s a reason He’s so obscured.

You can’t handle the truth and the way and the life!

I wanted to link to his profile too but he seems to have delete his LW account. :(