I think it would be fair to say that "understanding" requires some sort of internal state change. We can definitely talk about how we did not understand in the past, but now we do. GPT-3 can't experience a state change, by its design.

I don't have to be able nail down exactly what kind of state change "understanding" is to point out that the way we use the word clearly entails one, and GPT-3 clearly can't have one, so therefore it is quite reasonably excluded from "understanding".

(In theory GPT-3 could be extended by continuous online training, but my impression is that convergence is too slow for that to be practical, and it wouldn't look much like "learning".)