Well thats certainly true, but from the perspective of undergraduate me it didn't seem that way. Admitting ignorance seemed to me like it would come with a loss of respect.
While teachers and lecturers certainly prefer their students to ask questions etc, I think plenty will also be consciously or unconsciously keeping a log of who seems to understand things more quickly and so on. While asking questions is what you're meant to do, and will almost always be of the greatest benefit to you as a student, the fear of losing respect isn't entirely unfounded.
I think it's very hard to be sure that you as a teacher are avoiding all judgement of this sort too, as avoiding all such judgement is both the sort of thing you professionally...
Reading Being a teacher made me think about my experiences tutoring university students. I'm a PhD student, so my teaching currently consists of helping first year undergraduates on problem sheets. I think I'm reasonably good- I try to appreciate what difficulties they are having and anticipate them, by explaining what they are doing means, and approaching the problem in different ways.
One constant frustration I get though is that, having explained a problem to a particular student, the student will give me a blank look, and then mutter "ok". I know what that look means, and will ask "so do you understand that?" "sort of...." "well look at it this way....".
Now some of this may come from me- I'm explaining too fast or in a way they don't understand, and my familiarity with the subject, but I suspect some comes from them. It can be difficult to admit one's ignorance, from my own experience. I, and I suspect others who go on to do university maths, was used to being the best or near the best in school, with "being smart" being part of my core identity, something that made me distinct from my more attractive or more fit peers. Getting to university and realising one is having difficulty with even basic questions can be a knock to ones identity. So I hid my ignorance, and did myself damage. I might lose the respect of a tutor, or even a lecturer by admitting my ignorance, but the alternative is to remain ignorant.
I suspect this is a problem that is common among us all. Its a lot easier to pretend we understand, and sometimes it may help- if we want to impress a potential employer we shouldn't admit ignorance (unless the alternative makes us look more ignorant)- but in general admitting ignorance helps us learn. There is (almost) always someone with more knowledge on a particular subject than you, and a failure to use that resource is a failure of rationality.