Inevitably WB will get questions based on false assumptions and all sorts of sloppy thinking. Any question about the evolution of space whales or space-based lifeforms is almost certainly guaranteed to get steam pouring of out my ears. Many WBers will throw the book at anyone who dares ask about any possibility of faster-than-light travel or communication. It's guaranteed grounds for closure.
Both are common tropes of science-fiction of science-fiction worldbuilding. I prefer to look away and let the questions be answered. WB SE allows questions about magic and gods, are these based on false assumptions? Certainly they are poorly supported by empirical evidence.
The real problem with closing questions based on false assumptions is how do any of us when for absolute certainty when a set of assumptions are false? My call on what is a false assumption will be different from yours, and vice versa. What if what we call a false assumption is based on knowledge beyond what WBers know to be true?
Early in my time on WB I saw a question closed as opinion-based. The question could have been answered because there is an obscure area of theoretical physics that deals with exactly the subject of the question. If there was a false assumption closure rule it could have been shut down with that too.
What about the circumstances when an OP wants to ask about an otherwise impossible piece of worldbuilding. For example, how to conceptually construct a flat earth or a hollow earth. And why not? This is worldbuilding after all and if we can't work with the impossible, what are we left with? Probably, very little.
As an exercise try running through recently asked questions and make a list of how make false assumptions, i.e., assumptions that are contrary to quotidian reality. Once you've done that, ask yourself whose version of reality are we going to stick with. Certainly not mine, I'd want to close down space whales. (OK. I wouldn't actually, but I do prefer to look away from them.)
For me personally, many of the best questions I've come across on WB SE involved subjects or concepts my first reaction was that's completely ridiculous, then the cogwheels slowly started to turn and my thinking stirred as I found myself contemplating what I had thought absurd or impossible. Being made to think is the first task of any worldbuilder.
Also, in my opinion it should take more knowledge and careful argument to justify why an assumption is false than would be required to answer any question. A good answer to any question based on what, the answerer thinks is false, will be to disabuse the OP of their assumption(s) and point them in the direction of better ideas.
Arguing about which assumptions are false or not is quite likely to endanger this site's nice policy. Perhaps it would be more sensible not to go there.