HN is served by CloudFlare, and CloudFlare's edge servers should offer you an IPv6 address if you ask for an AAAA record - they certainly do for my domains - but it looks like someone hasn't clicked the 'sure, serve my site over v6' button in the CloudFlare UI.
If I run
dig -t AAAA news.ycombinator.com
I don't get any records returned, unlike every other CF hosted site that I tested. HTTP headers and NS records confirm CloudFlare is running things, just not offering v6 addresses...