Look for whatever category you think a comment on a thread on HN would fall into. I don't think that would be "formal" anything. You didn't specifically mention "informal written" but that is probably what I would classify it as since this is an informal setting and it was written not spoken. The top few results certainly put it in the "acceptable" category. General usage of the phrase in similar situations leads me to believe I don't really need to defend my usage of it.