Teaching is a skill on its own, it's orthogonal. I've read, and believe through my own intuition/experience, that being a "true expert" in most things is often such an intuitive place to be that it cannot be translated mechanically in that person's mind into steps for teaching it.
It's arrived at after years and years of "doing" and growing past internal boundaries and goals.
One thing you can be a "true expert" at is teaching itself.
Teaching relies on two things, primarily, beyond domain knowledge: making connections, and empathy. A great teacher understands not just the topic, but how it relates to other topics. Furthermore, a great teacher also has the ability to probe the person being taught to discover which connections are missing. Finally, of course, the connections have to be communicated, but communication skills are the least important. (If you don't have domain knowledge, if you can't make connections, and you can't get inside the head of the "student," you can't teach. If you can do all of those things, but you struggle with communication, you can eventually get your point across if you are determined).
It's the ability to make connections that we mostly associate with "understanding."
I'm suspicious of "true experts" who can't explain themselves. I think what a lot of people witness are people with deep domain knowledge ("experts") who just don't bother to make the connections. In my view, making the connections is a large part of being an expert.
