Interesting, I feel the opposite, I agree that an excellent teacher who knows the material back to front is invaluable.
My general model for how "being good at programming works" is that it's just mostly a stacking buff based on how much you've touched, I'm choosing to give the person in the anecdote the benefit of the doubt and believe in both their technical expertise and skill as an educator. Most technical things are kind of like other technical things, and if you've been around for a while everything is kind of like something you've done before, it makes it very easy to pick up new tools/domains. I fully believe that someone can open up a VAST gulf of knowledge of C++ between themselves and intro to C++ folks in a weekend if they're already a seasoned practitioner.
Nope, some superficially similar technical things are founded on very different concepts from other technical things. For example, grokking a functional language requires a whole different mental model than for an imperative language.
Also, remember that you could pick up a new language and start to dabble in it after a few days, but teaching it, ah, that requires much more than using it . Usually teaching something requires a much deeper understanding than just using it.
My general model for how "being good at programming works" is that it's just mostly a stacking buff based on how much you've touched, I'm choosing to give the person in the anecdote the benefit of the doubt and believe in both their technical expertise and skill as an educator. Most technical things are kind of like other technical things, and if you've been around for a while everything is kind of like something you've done before, it makes it very easy to pick up new tools/domains. I fully believe that someone can open up a VAST gulf of knowledge of C++ between themselves and intro to C++ folks in a weekend if they're already a seasoned practitioner.