He isn't, you are. Let's look at some other Feynman quotes which we can actually attribute (I can't find a real source for your claim though I've also heard it attributed to Einstein).
> Hell, if I could explain it to the average person, it wouldn't have been worth the Nobel prize.
> I can't explain [magnetic] attraction in terms of anything else that's familiar to you. For example, if I said the magnets attract like as if they were connected by rubber bands, I would be cheating you. Because they're not connected by rubber bands … and if you were curious enough, you'd ask me why rubber bands tend to pull back together again, and I would end up explaining that in terms of electrical forces, which are the very things that I'm trying to use the rubber bands to explain, so I have cheated very badly, you see."
> we have this terrible struggle to try to explain things to people who have no reason to want to know. But if they want to defend their own point of view, they will have to learn what yours is a little bit. So I suggest, maybe correctly and perhaps wrongly, that we are too polite.
My best guess is that this misattribution comes from quote ABOUT Feynman and a misunderstanding at what is being conveyed.
> Once I asked [Feynman] to explain to me, so that I can understand it, why spin-1/2 particles obey Fermi-Dirac statistics. Gauging his audience perfectly, he said, "I'll prepare a freshman lecture on it." But a few days later he came to me and said: "You know, I couldn't do it. I couldn't reduce it to the freshman level. That means we really don't understand it." - David L. Goodstein
Which doesn't mean what you're using your "quote" to mean. As I stated before, we (the scientific community), don't even know what intelligence is. We definitely don't understand it so I'm not sure how you'd expect us to explain it to an 8 year old. Lots of things can't be explained to an 8 year old. Good luck teaching them Lie Algebras or Gauge Theory. You can have an excellent understanding of these advanced topics and the only way that 8 year old is going to understand it is if they are a genius prodigy and well beyond a layman. This quote is just illogical and only used by people who think the world is far simpler than it is and are too lazy to actually pursue its beauty. They only want to sound smart and they only will to those dumber than them.
Stop saying this and get off your high horse and hit the books instead.
Maybe Feynman was right, we're being too polite. There are a bunch of people in this thread that are not arguing in good faith and pretending to be smarter than people that are experts and performing mental gymnastics to prove that (you are one but not alone). If an expert is telling you that you are using words wrong, then you probably are. Don't just assume you're smarter than an expert. You don't have the experience to have that ego.
https://en.wikiquote.org/wiki/Richard_Feynman
https://www.sciencealert.com/watch-richard-feynman-on-why-he....