Seems like it tests a limited list[0] of domains associated with the site or service, with the final result based on a check of their DNS records. For Steam, looks like it only checks <store.steampowered.com>, <steamcommunity.com>, and <help.steampowered.com>.

It'd be nice if the site provided an explanation of the process, maybe with some way to expand a particular entry and see which associated domains were tested and which succeeded/failed.

[0] https://github.com/andir/ipv6.watch/blob/master/conf.yaml

Like another commenter mentioned, it would be nice if there was a detail page showing what no, partial, or full support means. I plugged a few of these into https://ipv6-test.com/validate.php (a random site I found) and store.steampowered.com has no AAAA record, twitter.com has no AAAA record (this site says it has partial support), www.ubuntu.com has an AAAA DNS record, an IPv6 webserver, but not an IPv6 DNS server (this site says its supported).