>gzipping format that uses deflate compression
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(http://stackoverflow.com/questions/388595/why-use-deflate-in...)
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).
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.
To clarify the original comment: I think it's rather pointless to gzip a PNG file, since PNG files use deflate compression, the same method gzip uses, and hence it has very little real benefit, if any.