As evanrmurphy says, the citation is only there to link to the relevant HN thread; it's not implying that HN is (or isn't) a credible or reliable source. There's currently 57 external links to HN from the en.wikipedia.org domain,[1] though not of all them are used as citations.
Wikipedia's Verifiability policy indirectly comments on sites like HN here: http://en.wikipedia.org/wiki/Wikipedia:SPS#Self-published_so... (and the section directly below, "Self-published or questionable sources as sources on themselves"). So for example, a comment pg made on HN would be something that would be an acceptable citation since his work in the field has been published in numerous reliable sources in the past.
I'm not sure that's the same as calling HN a credible source, because they're not vouching for anything said here. In any case, it's interesting and I've never seen it before on Wikipedia. :)
Wikipedia's Verifiability policy indirectly comments on sites like HN here: http://en.wikipedia.org/wiki/Wikipedia:SPS#Self-published_so... (and the section directly below, "Self-published or questionable sources as sources on themselves"). So for example, a comment pg made on HN would be something that would be an acceptable citation since his work in the field has been published in numerous reliable sources in the past.
[1]: http://en.wikipedia.org/w/index.php?title=Special:LinkSearch...