Thanks for an important meta-question @uhoh, and especially for your comment on (@)Mick's answer. While your "insta-closure" label is a bit inflammatory, your comment is really helpful in understanding your position. I do get the point you're making. I also acknowledge your unstinting efforts at making newcomers to our site feel welcome.
I cut my SE teeth on [ELU.SE], which has very high traffic & an almost unmanageable number of genuinely bad questions - 150+ questions in the Close review queue isn't unusual. But you're quite right that the low traffic on [Astronomy.SE] doesn't warrant the same vigilance, and [note to myself ;-) ] we can therefore be more relaxed in assessing "borderline off-topic" questions.
We should remind ourselves of this site's purpose, which is set out in the opening paragraph of Astronomy [Tour] page:
Astronomy Stack Exchange is a question and answer site for astronomers and astrophysicists. It's built and run by you as part of the Stack Exchange network of Q&A sites. With your help, we're working together to build a library of detailed answers to every question about astronomy.
The last 10 words are the key, and should guide us every time we review a question. Is the question about astronomy? If not, it should be closed. If it is about astronomy (as defined in the On Topic page in the Help Centre), has it been asked before (close as a duplicate), and if not, is the question clear enough to enable our site's community to deliver an authoritative answer? If the question is unclear, or needs further detail, or is too complex (eg too many questions & needs more focus), we can either edit it ourselves if the OP's intention is clear, or we should post a comment drawing the OP's attention to what needs editing. And we can VTC in case the OP fails to edit the question appropriately.
The grey zone is typically where there's something in the question that causes some in our community to think it's either not about astronomy, or it's pursuing an unscientific path of enquiry. There is certainly a knee-jerk reaction to anything that seems like it's about astrology (mention the word "zodiac" at your peril!) or weird hypothetical (like "would the Moon still cause tides if it was made of cheese?", or "What do you think of my theory that dark matter is caused by ...").
The question you've linked to is one of these "grey zone" questions, as it refers to the idea that ʻOumuamua might be an alien spacecraft. This is no doubt what prompted the VTCs. Nonetheless, your answer to that question demonstrates that even if the question itself seems to veer towards being off-topic, a good answer can steer it back towards astronomy.
Referring back to my earlier quote from the Tour page: yes, this question does enable us to fulfil our site's purpose. Anyone searching our site for questions about ʻOumuamua will find a useful answer on how radio signals might be detected or received. Our library is improved as a result.
TL;DR: As a relatively young Stack Exchange site with a low volume of questions, we can afford to be a bit forgiving if a question strays into the grey zone, provided we can see the opportunity to steer it back via an authoritative answer using mainstream science.