This is my own opinion, but I think it depends on the role of the unrealistic physics in the question. I ultimately agree with the attitude expressed in JMac's answer on the fictional physics post you link to. If an answer to the question would rely on false physics, then the question should be closed as off topic. However, if the unrealistic physics/fiction is just being used to set up the scenario, and an actual answer to the question would then use legitimate physics, then I think the question is fine.
Of course, this is just my opinion. There are instances where just a fictional premise causes a question to get closed. One example can be found here. If you look back through the edit history, you will see that the question originally asked about a nonphysical scenario of the sun disappearing, even though the heart of the question was "how can angular momentum be conserved when traveling in a straight line?". The comments have since been deleted due to the question being edited, but there was much criticism due to the bad premise, and the question was closed as being unclear.
In general I think it is a challenging to try and determine hard and fast rules in scenarios like these. As you have shown, in the same category of "unrealistic physics" there are questions that remain open and get many up votes, and there are questions that are closed for not being about mainstream physics. I think this is an instance where the outcome greatly depends on which users view the post, how carefully the post is read, who ultimately decides to close the question, etc. Some users are more lenient on questions like these than others, and this could make the difference on some of the more "iffy" questions.
So ultimately, I think you just need to take scenarios like these on a case-by-case basis. Is the question asking us to imagine a world where there is no resemblance of physics as we know it? Or is it using an unrealistic scenario in order to explore realistic physics?
