Tbh this site is pretty misleading. Fastly and Cloudflare support ipv6 pretty well in their productы, it’s just that their marketing team apparently don’t care
The website is pretty explicit about that it's testing if a domain/website is available over IPv6 vs IPv4. Clicking on a website also sends you to https://ipv6.fail/domain/cloudflare.net/ where it's even more explicit what they're testing against. Wouldn't call it misleading exactly.