That is hopefully obvious to anyone that had a decent education.
That learning through teaching works is also obvious to anyone who tried. I didn't know Feynman advocated for this, but I discovered it when I was an undergrad. To prepare for (oral) exams, I would find an empty classroom and repeat proofs and theorems and explain concepts to an imaginary audience. It felt easy and forced me to clarify the logical flow of everything I studied.
Another aspect is when I was giving private lessons to younger students in need of help. This was very cool because I reviewed subjects that I wasn't quite ready to understand when I first studied them. For example when teaching thermodynamics to a guy, that was the first time I really felt I had understood it properly, because by that point I had other tools (statmech).