I think rationalist cults work exactly the same as religious cults. They promise to have all the answers, to attract the vulnerable. The answers are convoluted and inscrutable, so a leader/prophet interprets them. And doom is neigh, providing motivation and fear to hold things together.

It's the same wolf in another sheep's clothing.

And people who wouldn't join a religious cult -- e.g. because religious cults are too easy to recognize since we're all familiar with them, or because religions hate anything unusual about gender -- can join a rationalist cult instead.