That link is what are normally called vanity nameservers. That allows "branding" them so that "dig NS foo.com" says ns1.foo.com and ns2.foo.com instead of pinky.ns.cloudflare.com and brain.ns.cloudflare.com. But CloudFlare is still the provider.
What you cannot do is set your domain's nameservers to some other other provider, e.g. Route53. There's just no option to tell the registry you want to use non-CloudFlare backed values (outside of "call us" enterprise plans).
This is particularly nefarious when combined with buying a new domain. New domains can't be transferred to another registrar for 60 days, so if you need a DNS feature or config CloudFlare can't provide on a domain you just bought, you're just totally stuck for 2 months.
What you cannot do is set your domain's nameservers to some other other provider, e.g. Route53. There's just no option to tell the registry you want to use non-CloudFlare backed values (outside of "call us" enterprise plans).
This is particularly nefarious when combined with buying a new domain. New domains can't be transferred to another registrar for 60 days, so if you need a DNS feature or config CloudFlare can't provide on a domain you just bought, you're just totally stuck for 2 months.