HN uses Cloudflare (a YC company) and could turn IPv6 on for free with a single click.

But that may not represent the full story for them. Internal moderation and anti-spam may need updating to be compatible with running on two different networks that each have different approaches to numbering.

Even companies that 'get' v6 sometimes have rough edges. E.g. Cloudflare - which has been a great supporter of v6 for a long time now - has been known to send 'log in from a new IP address!' notifications because my machine automatically rotated it's IPv6 privacy suffix.