As much as I'm a fan of Gwern's work the conclusion he reaches is dead wrong. A high rate of attrition does not prove that brain washing does not work. It proves it works as long as a victim can be kept isolated, see previous comment about how cults do what they can to isolate their followers. The converse is also true, family and friends already belonging to some group is a powerful agent to keep an individual
in that group.
So all of this is about belonging and how to get someone to move from one camp to another.
Scientology and all the other cults out there have to live by the same sword that they die by: influence. And the fact that there are online support groups for some of these cults is a powerful anti-dote.
Some groups are simply more effective at isolating their followers from people who really have their best interest at heart.
I'll do you the courtesy of continuing the conversation on your terms, you want to call this brainwashing and if that's it then it works. If you want brain washing to refer to the CIA being able to program people then that indeed does not work. It requires constant upkeep and maintenance as well as the will of the victim to belong to some group to begin with. But once that condition is met the victims can be made to do a lot of stuff they would not otherwise do.
If you have not seen firsthand how cults can take otherwise normally functioning people and drive a wedge between them and their loved ones to see their lives taken over then I don't think your - or Gwern's - faulty conclusions count for much. It's bad enough that this happens, it is much worse to see people deny it because that essentially leaves the victims of cults without any support at all, which effectively enables the cults.
For many people - including many people who are simply religious - fear of being thrown out of the in-group is sufficient to get them to act against their own self-interest.
Quite a few religions have this practice and members have been known to kill themselves for the simple reason of being thrown out of the group they've been a part of for all of their lives simply because they are who they are. To see you dismiss this so callously because of a link to a narrowly researched article that has a conclusion that skips a few thousand years of evidence to the contrary seems a bit of a short-cut.
Take for instance Catholicism and the fear of excommunication, a powerful form of peer pressure where the people thrown out of the group are shunned.
https://en.wikipedia.org/wiki/Excommunication
here is another interesting view on the original subject, eating disorders and the kind of pressure they flourish under:
http://nedic.ca/eating-disorders-and-depression-story-surviv...
Not to mention the kind of pressure your typical ballet dancer is under (or fashion models for that matter).
None of these rate a mention in your finely researched link, and that is the sort of thing I was getting at before you moved the goalposts.
Isolating people from their friends and family is a tried and true method of cults (including mainstream religions) the world over. It usually starts with attacking the identity of the person and driving a wedge between them and their families or friends. Can't have that kind of interference if you're going to get away with the next stage.
That's a very practical approach. For an encore, of course it helps to start with targeting people who are already in a fairly weak position socially.
For more examples of how strangers get into your head and make you do stuff you don't actually have a self-interest in see advertising, aka applied psychology. Attack the persons self-image and presto, you can sell beauty and fashion products much more easily. Buy our shitty product and you too can be loved.
Of course in basis everybody has agency and you could if you are really strongly opinionated resist all of this. But even for the most die-hard resisters the fact is that all of this works, and works ridiculously well. The best way to stay on the right side of the line is to avoid exposure and that is getting harder and harder.