First, mod stuff:
"The current set of answers are speculative but it feels like this question has been pre-emptively closed by a moderator without allowing the rest of the community a chance to consider whether or not it is opinion based."
The problem here is that you don't understand how closure works.
A question can be closed or opened by any 5 users in sufficiently good standing. Closure is easier because of the way traffic works, but the system itself is fairly symmetric.
Additionally, special users with special gold tag badges can close or open things with a single vote for being dupes. Maybe someday they'll get to do it unilaterally like mods, but also maybe not.
Also additionally, black-diamond mods can open or close questions with a single vote.
When a moderator closes your question, that doesn't mean it can't be opened again. In this case someone VTCed your question, left a reason, and we disagreed with that reason so we reopened it. That's fine behavior. The fact that the person who thought your question should be closed was a mod is irrelevant.
In order for SSD to be '[not] allowing the rest of the community a chance to consider whether or not it is opinion based' he'd have to do something like post a comment saying "don't reopen this question because we've decided so cause INCOHERENT POWER TRIP" or something. That's not something that's ever going to happen because, unlike certain other democratic processes, we only elect people competent enough to understand the very basics of the powers of the office they've been given. If the mods wanted to close a question without community review they wouldn't cast a close vote: they'd delete it. Or lock the post. Or any number of other things that regular users can't just override if they disagree.
This isn't a problem, no malfeasance occurred, and the assumption of malfeasance was overly uncharitable. Having less power in a system than someone else can be frustrating, but that doesn't make it okay to be mean to people. Remember mods are people too. This is especially directed at the people posting uncharitable and inappropriate things in the comments on SSDs answer. The OP's post is misguided and belies frustration/upsetness at perceived failings in the system, but is not unacceptably uncharitable, at least by my reading.
Actual question time
Speculation is kind of a bad word. In modern times it specifically refers to bad reasoning, so answers that are 'speculation' are automatically bad by definition. It's true that speculation isn't necessarily opinion, but it is necessarily baseless and unsubstantiated in critically important ways. Lots of people believe that bad baseless claims and opinions are interchangeable. I would agree with you that they are not, but that is not really relevant to whether or not speculation is okay in answers.
Our current site policy is that it is okay to close questions based on the answers they are getting rather than the question itself. That policy is something we could change if we decided to, but that's something to discuss elsewhere (i.e. another meta) if you are so inclined.
The network's policy is that close reasons are a courtesy, and don't need to match up with the problems a question has in actuality. This is problematic and not a good policy, but it's the way things work. This means that if your question is closed as unclear and you edit it so that it's now clear but way too broad, we don't change the close reason to 'too broad', we just leave it closed as 'unclear'. This means that even if 'Opinion Based' wasn't the best reason to close this it shouldn't be reopened just because it's not 'Opinion Based'-- you'd need to wait for people who think the question really should be opened.
That said, Opinion-Based was the wrong close reason. The right close reason, if we agree with SSD that the question is drawing too much speculation and should be closed as a result, is "This question is drawing too much speculation and should be closed as a result". Unfortunately, we only have the following options to pick from:
- off topic
- custom reason! But as a subcategory of off-topic...
- too broad
- opinion based
So if we've decided that 'is drawing bad answers' is the reason it needs to be closed, we have to cram that into one of those reasons. Opinion based and too broad are the most wibbly wobbly of the reasons, and so those are the ones we (ab)use when we have to cram in a reason that, while possibly good or even site-required, isn't on the list.
Anyways, that's my take on why 'opinion-based' happened here, and why really that was probably the most appropriate reason to list.