Are questions about local dialects necessarily "opinion-based"? (If not, I believe my question was closed without proper justification)
Questions about local dialects are not necessarily opinion-based. However, you're in a looking-glass world here at ELU:
Questions are considered opinion-based if they're likely to attract answers based on personal, rather than 'expert' opinion.
Your question may be considered likely to attract answers based on personal opinions, primarily because you either haven't done any basic research to answer your own question, or if you have done such research, the question doesn't mention what (exactly and specifically) that research was and why it wasn't sufficient to answer your question.
I'm not the originator of the guidelines behind the 2 points I mention, nor am I able to justify or explain the reasons for them. I have merely observed the effects in operation.
The two points can only be considered guidelines. More or less obviously, some questions cannot be adequately researched by the people who have the questions. Equally, the points are only considered operative for a subset of questioners and answerers: some of the participants here are considered 'above the fray' and so are given special privileges and considerations. The members of that privileged subset appear to me to have been chosen by 'behind the scenes' activity, in the chatrooms and presumably elsewhere, along with longevity on the ELU site in conjunction with general acceptance of whatever elaborate fictions those privileged members choose to promote about themselves and their 'expert' status.
Anyway, I've voted to reopen your question because the research necessary to answer it is somewhat daunting, and the applicable resources are not generally or easily accessed.
As is, that question is totally on-topic. It is asking about the regional dialect meanings of a word. What's more on-topic about that? It is not asking for opinions (or if this question is then -all- questions on Stackoverflow are opinions - just because people might disagree doesn't mean they are opinions).
It might arguably be closable for not presenting say a dictionary definition of 'yankee' which includes the first two items and some claim that you cannot find any evidence for the last three. But that is a nit-picker's close reason... if you really feel that is necessary it'd be easy enough for the close-voter to instead do the edit themselves.
The text does sound like a fabrication though - the last line sounds too much like a punch line in its seeming absurdity. Either way, it is certainly so specific (and otherwise not publically documented) that it warrants a question on ELU.
It may be relevant that the existing menu of possible reasons for closing doesn't very well articulate all the reasons that may actually lead people to vote to close. It thus often happens that somebody feels that a question should be closed, clicks 'close', and then selects whichever reason seems to be the closest, even if it doesn't quite reflect the real reason. This leads to the posted reasons for closing being confusing and misleading.
In this case, it is possible that the last line of the quoted text led some people to perceive the whole text as a joke, and that they then voted to close the question because they treated it as a request that the joke be explained. As there is nothing on the menu that quite captures that reason, they selected 'opinion-based' as the closest available option.
Perhaps it is time for the established contributors to this site to reflect on how frustrating and alienating to questioners it is when they see their questions closed with posted reasons that make no sense. I realise, though that revising the menu of the reasons for closing may be a daunting project.
I scanned the Oxford English Dictionary full definition of Yankee, and it supports the first several definitions given in the @MaxB question. However, I did not see any mention of Vermont or pie. The OP probably does not have to go to the OED to find the non-Vermont and non-pie answers. So the question really reduces, after summarizing some research, to about Vermont and pie, and an answer from a bona fide long-time inhabitant of New England would be useful.
That is, answers based on authentic personal experience are useful even if not supported by a quotation. I remember fascinating answers from people who reported on the dialects of their local areas or language spoken by their grandparents, although I cannot easily find an example.