Please engage with the comment and ask for clarification if there's something you think is unclear. If you find something hard to believe, say so, and why. If you're just genuinely interested, ask for a book recommendation. But please write a full sentence.
What is being asked for is a source for the parent comment. You say you're tired of the single-word, zero-effort "sources" or "citation" meme. I can't follow that reasoning. Minimally, as I work in academia, I find the request for a citation normal. But also, I cannot agree with your opinion that only those comments can be valid that require from the author an effort that is greater than what you define as zero. I would think that your parent is actually interested in engaging in a discussion, but would like to do some background checks first - hence the request for sources.
I don't find that asking for a source is creating a hostile environment, either. You've summed up possible reasons why someone might ask for sources in your second paragraph, so I have no doubts you understood the motivations of your parent.
It's not quite clear to me, whether you're complaining about the content of your parent's post (that they are asking for sources) or the form of the post (that it's just a one-word post). In any event, I don't think it's fair to downvote someone because they ask for sources, just because that request was not written in a way that pleases you.
EDIT: On second thought, I feel my comment is a bit wanky. I wish I could re-upvote you, I think I acted prematurely. I'm sorry. :-/
First point, HN is not academia, it's (mostly) intelligent people discussing interesting subjects. In such discussions, it's perfectly fine to make contributions that you can't immediately source. It's, of course, also fine to contest those, and if someone says you're wrong, and has sources, well, you should be prepared to concede the point. But even in academia, I strongly doubt that you've ever received a request for sources from a perfect stranger that didn't explain exactly which point they were interested in.
Second point, form and style is important. Politeness is important. They are cornerstones of civil discourse. If we don't maintain a civil environment, intelligent people leave, and HN turns into the regular echo chamber that are par for the course on the Internet. Intelligent people interested in real discourse can and should explain their request when asking someone else to elaborate.
Third point - yes, I could guess at a range of possible motivations, but I don't actually know which one it is. The range goes from "I think you're wrong and probably an idiot, but I'm not going to bother explaining why" to "wow, that's a fascinating thought, where I can learn more". Those are vastly different responses - one obviously has no place in civil discourse. My gut feeling is that one word requests for sources are more often towards the former than the latter, but obviously I can't know. Even if my gut feeling is wrong, it still creates grating uncertainly: Is this person a troll who's out to undermine my credibility on the cheap, or is it a genuinely interested person who just want to learn more.
Being terse is a good idea sometimes, but I'm not sure those times are as frequent as it actually occurs.
It's trivial to write something that's blatantly incorrect, to add some obscure references that sound good (they may not necessarily even be related, and if they're hard to access, nobody will bother to check them), and then to pretend that such claims have merit. Sometimes this happens unintentionally, when the referenced material is taken to be correct, when it actually isn't.
A lot of people here and in other online discussion forums are hostile to the "citations please" attitude because these people have experienced academia first hand, and know that references are often quite worthless. In many fields, they're more about adulating prominent members of the community than they are about making claims more robust. It ends up being a wasteful activity to engage in, for the most part.