> It's completely typical to expect someone who makes a surprising factual claim to back it up with evidence.A citation does not necessarily provide evidence, it only guarantees the words of a third-party. A third-party can spout complete nonsense as well as anyone else. The request ultimately serves to appeal to a flawed assumption that a third-party's words are more valuable than the words of who you are speaking to.
Which also incorrectly assumes there is someone else's words to even draw from. HN is well known for attracting experts in their field. It could have very well been that the parent commenter is the only person who knows anything about the subject. There may not be anyone else. Even if there was, to dismiss his knowledge as the hypothetical leading expert to hear from some other random nobody doesn't make sense.
But, to give the benefit of the doubt, if we assume there is a greater subject matter expert who can give you better words than the parent to describe the knowledge you seek: What purpose does the middleman serve? Why not talk directly with the expert? You are going to get a lot more out of it. The middleman, if not a valuable party to the subject, isn't going to relay what is most useful if for no other reason than because he doesn't know what is useful. A citation remains pointless.
> You're talking to them because you think they might know something you don't
Of course. Which, again, questions why you would want to defer to a third-party? If you have good reason to believe someone knows something you don't, why wouldn't you want to hear it from them directly in their own words? As I said before, in this case elaboration is what would be valuable. A citation is not. A citation is completely useless here. While citations do have a place, asking for a citation in the middle of a conversation is a fallacious device.