to me it seems that its very apparent "i dont understand" is not a question. that is completely logical and there is no way it is ridicule.
dirac may want to reconsider his task of instructing if he views those that dont understand as part of the less intelligent, dense group who misses his subtle ridicule. obviously, its not teaching them anything. he and his fellow more perceptive friends may be having a laugh, but its not accomplishing anything.
I don't think dirac was ridiculing anyone.
you can have me memorize all of the mathematical proofs and derivations in the world, or the name of a bird in all languages of the world, but when I'm finished, I may not understand the proofs, or know anything whatsoever about the bird.
this, I think many scientists realize. and you see it marked through the use of analogy, through the demeanor of a sociable, kind person. being able to transfer a concept to a subject is far more a mark of intelligence than being able to regurgitate facts to them.