You now get charged for ipv4 addresses as well. Less than nat, yes, but now ipv4 addresses are no longer free either.
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.
I'll be sticking with ipv4.
Source: I have used NAT64 myself before.