> What gatherings?
Rationalist have groups meetup and conferences. For example [1]
> What beliefs?
It's pretty diverse and not uniformly held. Some of the more strange ones that seem to have pretty wide appeal are around AI safety. For example, Roko's basilisk.
> What proclamations? What evidence of insincerity?
I was responding to the comment above me that suggested that 95% don't actually hold rationalist beliefs and are instead simply having fun with it. Only a small 5% are actually sincere.
[1] https://www.reddit.com/r/slatestarcodex/comments/1car63w/les...
95% of rationalists always thought roko's basilisk was a joke
Okay, but those pieces don't establish your claim. You would need a Roko's basilisk-like concept that only 5% of the community agree with, and a "group meetup" or "conference" where "they all" (i.e.: also the 95%) "proclaim that their beliefs (i.e.: including the basilisk thing) are true".
If I observed such a gathering knowing that 5% of the people there have sincerely come to a particular conclusion based on the available evidence and the other 95% think the evidence doesn't support that conclusion, I would be extremely surprised to see everyone going around "larping belief". And I don't think you can actually show such a thing occurring.