First off, it's not clear how many of them did assume the false way - sure, there seems to have been a tradition of teaching by describing the movements the fingers are supposed to make in great detail, and it looks as though the teachers in that tradition were completely wrong about how they should move. I gather that that basically started with Shearer in the States - I wouldn't want to underestimate the reach of that tradition, but it doesn't represent the whole world of guitar teaching.
The videos of good players saying one thing and doing another show that even they are not that aware of precisely how their fingers are moving. This I think is one reason why it's a really bad idea to try to teach the movements directly, even if - thanks largely to the other thread - they are now much better understood. More to the point, it opens the way for trial and error. It means it's not really surprising if people who did come up in the Shearer tradition adjusted their technique over time and didn't realise that they had ended up doing something quite different from what they had been told to do.
As far as I can see, we learn practical skills by trial and error. The guidance you get from your teacher is only ever a starting point. You'll never get the hang of something by reading a book, even if reading a book can start you off in the right (or the wrong) direction. It's about practice and calibration.
What happens when someone has learnt to pluck from MCP at a slow tempo and then ups the speed, or attempts a tremolo? Probably they don't really know, except that it doesn't go that well at first. With a bit of practice it gets easier and the fingers start to find their way - that's just trial and error. In the process the fingers start to extend at the point of release instead of flexing, but practically nobody notices this, and even fewer people care.
The value in the new understanding is that teachers can use it to diagnose problems, design exercises, recommend studies etc. The danger in it is that people may think that it is a good learning strategy to focus on what their fingers are objectively doing, rather than the sound they are making and how the movement feels inside - that is just repeating the original mistake.
Someone who listens carefully to the sound they are making and also to their body will end up with effective technique. Someone who tries to make the precise movements described by a teacher will need a lot of luck along the way - even if the teacher is describing the right movements.