This is credible, but I'd guess that it'd be more accurately restated this way: if you recently learned/mastered a topic, you both have a fresh recall of the e

This is credible, but I'd guess that it'd be more accurately restated this way: if you recently learned/mastered a topic, you both have a fresh recall of the elements involved in reaching your level of mastery and what was involved in bridging that (personal) gap between unskilled ignorance and that mastery. Whether that puts one in a far better position to teach than an expert probably depends on the expert. I definitely had math professors for whom the charge applied, and I'd speculate that an expert who's primarily focused on further mastery/development along the edges of the field might be someone who becomes less and less able to recall or relate to what it's like to take first steps into it. But on the other hand, I had math professors who clearly thought a lot about pedagogy as well as research, and practiced both at length... and I think someone who's spent significant time teaching beginners and reflecting on that as well as practicing their discipline and refining deep insights from that is likely in a better position to teach a topic than someone who just learned. (And if you really want an optimal learning environment, put a class full of motivated learners in front of a teacher like that, and have them talk to each other. Hence the value of a good university course...)