That's oft cited but usually by people who know Feynman from the shuttle explosion. I've never heard a physicist cite it, and as a former physicist (high energy theorist) myself I have a few qualms with it.
I would say even the Feynman Lectures for undergrads aren't simple, but that's arguable, so let's talk about Feynman diagrams, a supposedly intuitive invention of Feynman. Pretty little line drawings of particle interactions are indeed useful for surface level explanation, but physics is quantitative (that's how we verify anything) and staring at line drawings gets you nowhere in that regard. So the layperson might ask, "how do physicists calculate anything with Feynman diagrams?" at which point Feynman himself would probably be speechless. The path integral calculations for the simplest case in QED is like a page long, which is complete gobbledygook for the mathematically unprepared (>99.5% of the general population), and even for the mathematically prepared, the calculations mean nothing without tons of theory buildup. I have on the shelf right next to me the classic tomes on quantum field theory: >800pp Peskin & Schroeder, >800pp Schwartz, three volumes of Weinberg, all filled with dense mathematics. You need to study at least about a quarter of those to properly understand the aforementioned topic, no to mention all the prereqs: various formulations of classical mechanics, some classical electrodynamics, quantum mechanics, special relativity. There's no shortcut, anyone claiming to have an intuitive understanding of Feynman diagrams (the real deal, not just the line drawings) by reading some "simple" explanations is lying, regardless of whether they have genuine interests. (Btw, the ones with genuine interests are sometimes the worst: the "my theories are correct, I'm just bad at math!" alt scientists who love to email their papers to our entire department.)
In conclusion, there are fields where there's really no way to explain things beyond the extreme surface level in simple terms, and ironically Feynman's own is one of those.