As asked in my flag, why is this question on hold? This is not primarily opinion-based as the hold states it is. It's asking for actual research. Also, why is my flag declined when there is legitimate reason for intervention for the content. Read the question. It's not primarily opinion-based. It requires intervention due to a failure in SE's "Theory of Moderation" mentality.
The touchstone statement of A Theory of Moderation is:
Moderators are human exception handlers, there to deal with those (hopefully rare) exceptional conditions that should not normally happen, but when they do, they can bring your entire community to a screaming halt — if you don’t have human exception handling in place.
(I can't tell for sure if you agree with this idea and think your flag signaled just this sort of exception or if you disagree with this policy. For the purpose of this answer, I'm going to argue this flag is not necessarily such an exception.)
I agree the question on its own doesn't ask for mere opinion, but rather data and "real-world experiences". The problem is that questions don't exist in a vacuum. The comment section has been cleaned up, but included such gems as:
Just another authoritarian indoctrination technique.
For whatever reason, readers didn't catch what, exactly, the asker was looking for. There are 15 answers and while some of them are based on real experience, others are not. The linked answer on Skeptics does a better job of bringing in research. So even though the question on its face is not opinion-based, it is attracting a lot of opinions.
Now there are several things that can be done to correct the problem:
Deleting comments and, perhaps, answers would be a good place to start. That's the sort of exception that does require a moderator to handle.
Leave a comment encouraging people to answer with data rather than opinion.
Edit the question to make its intentions more clear. I notice you did edit
, but managed to make the question more focused on opinion, in my opinion. [On second examination, your edit was helpful and I misread the diff output.]
Reopening the question as is won't solve the primary problem that people are using it as a platform to argue about broad educational philosophy. I wouldn't expect a moderator to reopen the question without doing something to mitigate the opinionated answer problem. I'm not sure your edit was quite enough to do that.