It was not me who handled that, but I have handled quite a few such flags in the past in the same wat. A flag is not binding and moderators can disagree with the judgment that the question should be CW.
I for one have found so far very few reasons to deprive answerers of symbolic retribution for their answers, and I have never so far turned a question into CW. That a question does not have a definitive answer does not mean in the least that producing one of those possible answers does not take effort, for example, amd much less producing a good answer! In the particular question you refer to, you can find pretty good answers into which the authors have put considerable effort—I would say that more effort than the average for a math.SE answer... yet the multitude of possible answers somehow makes their effort less valuable than this in terms of reputation points?!
The one useful thing about CW is that the rep requirement to edit other people's answers is lower, if I recall correctly—I have yet to see this have any effect in practice. Additionally, if a question is CW then the owner can not accept an answer without the cost that that usually implies in terms of the accepting percentage... but I regard that as a bug in the system in that it conflates two completely different things.