Generally when one refers to a country as being "occupied", the implication is that the people there are poor off as a result, especially relative to a neighboring country whose land is presumably being considered occupied, which in this case is implicitly North Korea due to the history of how the two countries split. It is therefore not in bad faith to directly ask whether the people there are better or worse off as a result.

You also seem to be hyper-fixated on one possible meaning of prosperous, which is "wealth and GDP", when there are lots of other means related to flourishing in general. The original commenter was perfectly free to provide an answer along the lines of, "North Korea is the more prosperous country because X." where X is a list of ways in which the people there are flourishing, and this would have been a valid answer (if not necessarily a correct one).

Alternatively, if they think that South Korea is better off but this does not matter because it is still less ethical than North Korea, then they could have taken the opportunity to be up front about that.

So in short, this question could have been used in all sorts of ways to provide an answer that clarified the commenter's position. It is a shame that we never heard from them exactly what their thoughts were.