Why do people always say "this is not reddit"? I never even heard of reddit till I saw those comments. And I think this observation made over and over again usually adds less to the discussion than the politically incorrect statements this comment is made in response to.
Also, I don't think your link relates the the comment. Your link is about choosing between gzip and deflate; the comment you found objectionable was about the irony of gzipping a file that is in a format that also support deflate (so you might compress a compressed file; which I think can actually make the file larger).
Many on HN were once / still are redditors. Especially proggit. The type of response where you quote part of the parent, don't add any comment or commentary, isn't especially helpful.
Addressing your second point: I also thought the comment was a snarky reply, wondering why someone would use gzip instead of deflate. The parent comment addressed, specifically, why they were using gzip on PNG, which is why I assumed the 3 character response was focused on the compression type, rather than whether to use it on PNGs.
Also, I don't think your link relates the the comment. Your link is about choosing between gzip and deflate; the comment you found objectionable was about the irony of gzipping a file that is in a format that also support deflate (so you might compress a compressed file; which I think can actually make the file larger).