Can a submarine swim?

If you define understanding as what a human does, then of course only humans can understand. What's the point?

You’ll notice that the definition of “understanding” is quite vague, and in particular it is untestable. In fact, I think we understand quite a bit less than we think we do. Grandmaster chess players will identify maybe 5 or so candidate moves in a position to assess, but the actual process of pruning the game tree is hardly deliberate. I assume Go is similar. Chess tactics are often “seen” quickly and subconsciously. Players are encouraged to practice chess tactics to train their subconscious in this way. Evaluating a position is an extremely qualitative exercise as well.

It seems that MCTS rollouts are a pretty good way of “understanding” a position. It is arguably one of the most unbiased ways. That fact alone suggests to me that it is at least very hard to judge. Maybe it is not interpretable by you or I, but I can’t say that rules out understanding.

I go back and forth on this. Machines aren’t introspective enough to have the kind of behavior most would call “understanding”. But then again, people aren’t either. And I don’t mean that in the most people are stupid but I’m very smart sense. I mean we quite literally don’t know why we have thoughts, we just do. We’re able to use words to provide justifications after the fact, but that’s not the same thing. The origin of any thought is a mystery to us.

shift a stone one position over and the Go AI can completely change its outlook, estimation, and probabilities.

Seems like it grasps the nature & significance of things to me!

it seems that way until the AI makes a mistake that even inexperienced players would be able to spot & avoid using traditional principles

Everyone makes "what was I thinking?" mistakes, and we should not conclude, from that, a lack of relevant understanding.

everyone makes mistakes, but not everyone makes the same mistakes. depending on their mistake, players very much can and do conclude a lack of relevant understanding. this conclusion is an integral part of the learning process.

Human players make blunders as well.

right, which is why humans do not claim to "grasp the significance" of their moves.