I think you're thinking of somewhere else. The up/down votes are a way of agreeing or disagreeing without cluttering up the comments with a bunch of "me toos" or "nuhuhhs"
I'm certain that I'm not thinking of somewhere else. I'm completely open to the possibility that I just remember it wrong, but I'm sure that it was HN that I was thinking of, and not another site.
There's never been such a policy on HN; you remembered it wrong, as have many before you. It's the same phenomenon as attributes pithy quotes to Einstein and makes Canadians think we have Miranda rights: people hang memories on the nearest pre-existing hook in the brain.
Nope. I've never spend much time on reddit, and I'm certain I've never seen that page before. Perhaps I am thinking of comments other people made on HN in the past (who thought that the policy was as I described).
Nope, I know I'm not thinking of Reddit (as I said in reply to another comment, I've spent almost no time on Reddit, and would not have seen their guidelines at all).