However, if you bracket out all the murders for a second (a big ask), I don't think there's anything particularly special about this cult.
This is just the modern incarnation of something that's been going on for almost a hundred years in California.
In California there are Bohemian social scenes with a mix of high-performing technical people, spiritual/New Age/woo people, and "human potential" secular spirituality and self-improvement.
Magicians and Satanists involved in the founding of JPL (with a cameo from L. Ron Hubbard.) in the 1940s and 1950s. EST seminars, Esalen, UFO movements, communal yoga groups. Lesser known is the "General Semantics" movement (really quite similar to the Rationalists) from the 1950s. All these movements mix and overlap.
Mostly these social scenes are weird but harmless. But genuine cults often form in them and then spin off.
The Rationalists themselves are just another incarnation of this. They may seem totally bizarre, but if you just dig back in the history of California, they look very familiar. They are part of this stew of self-improvement/spirituality/tech/meditation. In particular they are the secular self-improvement people -- one thing they did is run training seminars where they teach you new thought techniques that will help you be more productive and successful. Many of them also engage in Buddhist meditation practices or take psychedelic drugs for exploration. In that scene, cults form. There have been multiple other rationalist-adjacent cults, although those did not kill people thank God.
I think it's a mistake to start analyzing the rationalists' beliefs about Bayesianism and expected utility and all that, to try and wonder how a cult could have possibly formed in this social scene. Nothing should be less surprising.
Now that this cult went on to commit multiple murders -- this is surprising and frightening. Perhaps some of the rationalist beliefs have something to do with it, but since there are a small number of other cults that have committed murders, looking at shared factors across all of them, such as charismatic psychopaths as leaders, may be a better choice.
Edit: upon reading to the very very bottom of the article, the author makes the exact same point. I'll leave this up just in case others aren't good readers either.