> overestimate how well they learn things without the feedback of formal education

People benefit greatly from external human feedback, but 1:1 tutoring (with a good tutor) is roughtly an order of magnitude more effective and efficient than a classroom, and classrooms themselves vary wildly; in general the more participatory the better.

Self learning without any help can be more effective and efficient than a typical classroom, because as it is self directed, self paced, can use the best materials someone can find and a whole variety of them vs. whatever was assigned, etc., but it takes a lot of skill (which comes from past practice) and effort on the part of the learner. Many researchers/scholars are doing significant amounts of self-directed learning as their career, and can get very good at it with decades of practice.

The speed of learning in a classroom with 15+ students is inevitably unbelievably slow, and the time is used incredibly inefficiently. This isn't any particular problem with specific teachers, but as far as I can tell an inherent problem with aligning a bunch of people's preparation, motivation, and attention, and a problem with giving feedback in a 1:many setting. Teaching/learning speed generally seems worse at a primary school level vs. at a college level, given a very good teacher in both; I'm not sure if this is because of better prepared (because older) students, or just more motivated students.