Is there an established community standard for what 'opinion' is vs. 'theory?'
No, even whether a question is primarily opinion-based (POB) can be subjective. Of course, in some cases a question may be very clearly opinion-based. In the question that you cite, I think you can make both arguments. Let's do that to see what might drive close-voters and voters to reopen.
The question body ends with the following question:
Why is Biden's approval so polarized while certain people are able to escape polarization at least partly, even at the federal level?
One the one hand, you could say that it's opinion-based because it's hard to answer based on knowledge that's commonly available. To answer the question probably requires original research. Another view might be that while it's okay to ask for specific polling results, some polling research might be inherently flawed because of the way the research might be conducted. See for example Yes Prime Minister's bit on leading questions in polling.
On the other hand, you could say that the popularity of the US president is such a big subject that there is enough research available to answer the question without original research. And while some studies may be flawed, there could be more studies on the same subject which may confirm of dispute the findings of other studies.
Whether the reasoning toward closing as POB prevails over the reasoning to keep a question depends very much on the question itself. For now, we let our users decide. There are some limitations on closing and reopening, I think a user with the privilege can vote to close once and vote to reopen once (not sure about the exact rules), so it will settle somewhere eventually.
If you think a closed question should be reopened or an open question should be closed then the easiest thing to do is to vote for that action. Additionally, if you think it's not clear why your action is the right one then it's helpful to add a comment under the question explaining the reason for your action. If that doesn't work you could start a discussion about the specific question here on meta. Of course starting a whole meta discussion for a single question is quite a lot of work, but it's possible if you feel strongly about it.
Unfortunately, it's very hard to rule on the general case of POB (i.e. this meta question) because the circumstances vary from question to question.