As a teacher - not of dhamma, of course - I've got to say that every teacher must
start with his/her students' existing knowledge, skills and aspirations. Starting anywhere else will leave students completely adrift or bore them silly or fail to connect in some other way and they will (a) get nothing out of the teaching (b) not come back and (c) often be turned off the subject completely - as in, 'Yeah, I went to a meditation class once but it was a load of rubbish - all they did was chant in some weird language.'
So whoever is teaching a walk-in one-off class has to assume some degree of goodwill and a large degree of ignorance, and take it from there without teaching anything that is actually wrong
. Anything that has to be skipped over in this process can be taught later, so long as the students get enough out of the first class to want to continue - and, if we value what we are teaching, what we want most of all is for them to continue.
Teaching such a class becomes a juggling act: how much needs to be said about background and fundamentals, how much attention can be given to an informed questioner without losing the rest of the group, how much can a topic can be simplified without actually misleading the students in a way that will become an obstacle later. A teacher who really knows his/her subject will manage it better than a one who doesn't, but experience of similar sessions plays a large part too and so does sheer teaching skill.
I wouldn't presume to say whether KB's teacher knew his subject or not on the basis of the incident in the OP, although I do think the question may have been handled better. Something like, 'Perhaps we can talk about that afterwards,' might have been the best response, but such things are very situation-specific.
Every time I work with a new bunch of students, I do it a bit differently: they
are different, I
am different and the setting is different. And after every time I do it, I think, 'Hey, it would have been better to say X at that point because it would have been a better lead in to Y,' or some such thing.
The other thought that came to mind while reading the thread was an image, a visualisation, of the learning path which I like and which may resonate with some of you. I find it helpful to think of it as a spiral staircase, not a straight one: progressing from one step to the next takes you around in a circle, and you reach the same point again but at a higher level. For instance, Right Speech supports and encourages Right Action and that encourages Right Livelihood; that improved ethical base supports Right Effort and Mindfulness which refine Right Views and Right Understanding and strengthen your Right Intention ... and with all that in place, you will be even more more mindful of Right Speech and Right Action, and so it goes.
Why do I bring that up here?
Simply because anyone can step onto that staircase at whichever rung they are level with at the time, and eventually their knowledge will be completed.
In keeping with the first part of this post, I'd better say right now that what I have I have could certainly be said better, but it's the best way I can say it right here and now.