So I am going to go off in the opposite direction and say that often people who apparently can explain things clearly and concisely, using pithy aphorisms even, do not really have any deep mental model at all. I'll quote a poem for you, that I feel is apt here:

    Turning and turning in the widening gyre
    The falcon cannot hear the falconer;
    Things fall apart; the centre cannot hold;
    Mere anarchy is loosed upon the world,
    The blood-dimmed tide is loosed, and everywhere
    The ceremony of innocence is drowned;
    The best lack all conviction, while the worst
    Are full of passionate intensity.

I feel that many deep concepts are the same: the best practictioners might lack all conviction. They cannot distill any articulation. They have nothing clear to say that explains their understanding.

In fact, when I've pressed for some, they have been frustrated.

All of my gurus are the same. They simply cannot explain what they know clearly and concisely.

Is it because I only go to Gurus for information that is inordinately difficult - that, I can find the simplest explanations wherever, so those are not what I ask about? Perhaps.

But as a metric, I simply have not found people's ability to express concepts clearly, or to distill concepts, a good proxy for their understanding them; nor of their level of insight; nor of their mastery of the domain.

In fact, I can almost go as far as to state the opposite: the only people who can clearly explain things, are people who are completely wrong about them. (This is an extreme position and I probably am going a bit too far in this direction. I may not actually believe the sentence I Just wrote.)

Basically I completely reject the premise - I think we completely disagree about whether it is a useful or good heuristic, or even correlates in the positive direction!