I would say: You either don't understand your subject, or don't understand your audience, if you can't explain your subject to your audience, at the highest level they can understand, coherently.
The average person can understand anything ... at some level. Being able to match that level is positive evidence (but not proof) of competence.
Duality: Being unable to match that level is positive evidence (but not proof) of incompetence.
I suspect Feynman actually haven't been to the world of the middle-to-bottom sections of the bell curve, where that thinking becomes toxic. It only works because there's collective illusion that minority theorizations can be more correct. Absent that, or that inverted, majority becoming assumed likely more correct, not only one's explanations will be interpreted biased as likely more wrong, but acts leading to majority groups following your round-Earth hypothesis can be seen as manipulative and/or fraudulent. That kind of people(which exist) abusing versions of those lines get annoying fast.
The average person can understand anything ... at some level. Being able to match that level is positive evidence (but not proof) of competence.
Duality: Being unable to match that level is positive evidence (but not proof) of incompetence.