In those words? Yes. You may note that those are different words than Alicorn's, or any of mine.

ETA: Wow, got seriously ninjaed there. I'll expand. It's not the "I don't consider this a cult" part of the message that'd make me update away from the surface meaning so much as the "...and I expect you to get the joke" part. That trades on information, even if you don't know it, that the speaker expects you to know. The speaker believes not only that they're not joining a cult but that it's obvious they're not, or at most clear after a moment's thought; otherwise it wouldn't be funny.

Even there I wouldn't update positively on the words, though my prior would be much higher thanks to the circumstances. People don't use negative-valence words unironically to describe groups they're part of and aren't disgruntled with, and "cult" is very negative-valence; "I joined a cult!" [light, smiling], therefore, strongly signals irony.

The message is "I don't consider this a cult, and I expect you to get the joke". Contra Alicorn, I think it would have worked the same way for just about anyone -- the speaker would have to be cartoonishly clueless for it to be taken at face value.

Tone matters a lot here. "I joined a cult!" [light, smiling] is all but certainly a joke. "I joined a cult" [anger] probably means "fuck off and stop prying into my private life". "I joined a cult?!" [horrified realization] likely points to a group with some unpleasant practices, if not necessarily an actual cult, and an update in the positive direction wouldn't be out of line.

Online, of course, there is no tone, and so we have to go by context. "I joined a cult" in the title of a Reddit AMA means something very different than if you're talking about, say, GameFAQs.

Back in college, I occasionally used something like "...also, I joined this cult" as a joke on the odd religious inclinations of the people I was hanging out with at the time. (Several of them belonged to various esoteric strains of reconstructionist paganism.)

Probably needless to say, there was no cult, the joke was universally understood as a joke, and I remained atheist.

People also tend to blab and joke about things that put them in stressful situations. Joining a doomsday cult, or a cult which is thought by others to be a doomsday cult is something which can put a lot of stress on you. So it's not implausible that someone would say such a thing and mostly mean it.

People who aren't joining a doomsday cult wouldn't say that, either.

They might out of jest.

For instance, when I explained my change in life plans to people who are very familiar with me, I was able to use the phrasing "I'm dropping out of school to join a doomsday cult" because I knew this sounded so unlike me that none of them would take it at face value. Alicorn wouldn't really join a doomsday cult; it must be something else! It elicited curiosity, but not contempt for my cult-joining behavior. To more distant acquaintances, I used the less loaded term "nonprofit". I couldn't countersignal my clever life choices to people who didn't have enough knowledge of my clever life choices; so I had to rely on the connotation of "nonprofit" rather than playing with the word "cult" for my amusement.

I'm not sure this is a very good example. The reason that saying "I'm dropping out of school to join a doomsday cult" works is that people who are really joining a doomsday cult wouldn't say that. Acknowledging that you are aware of the phenomenon of doomsday cults is an effective way of signalling that you are not in fact falling for the recruitment tactics of such a cult and does not require the person you are talking to to know anything much about you personally.

If anything this is a more effective tactic when used on relative strangers who do not know you very well and might think you actually are joining a doomsday cult if you just tried to describe what you are doing in a few short sentences. This seems more like an example of straightforward signalling that you are fully aware of the existence of doomsday cults and so people can assume that you have not in fact been seduced by one.

To a first approximation I don't think it makes a difference, but it does add some logical uncertainty. Also, intuitively I want to be able to use anthropic reasoning to say "there is only a tiny chance that the universe would have condition X, but I'm not surprised by X because without X observers such as us won't exist", but I think doing this implies I have to give a different estimate if red = bomb.

I think this particular example doesn't really exemplify what I think you're trying to demonstrate here.

A simpler example would be:

You draw one ball our of a jar containing 99% red balls and 1% silver balls (randomly mixed).

The ball is silver. Is this surprising? Yes.

What if you instead draw a ball in a dark room so you can't see the color of the ball (same probability distribution). After drawing the ball, you are informed that the red balls contain a high explosive, and if you draw a red ball from the jar it would instantly explode, killing you.

The lights go on. You see that you're holding a silver ball. Does this surprise you?

It seems you have just closed the middle road.