This can’t be true. I always setup Cloudflare’s NS records right away when registering new domain names. I don’t even have an alternative DNS server. Cloudflare just can’t copy records from the old server in that case, but adding the domain to Cloudflare works fine.
All of my domains have used the same name servers and I know for a fact that I have changed them prior to adding a site on most of the domains I own. However, reading through the docs I did see that what I have been doing is not best practice and will refrain from doing so in the future.
However, I am still unclear why this site is being a pain to add. I have created a ticket with my registrar providing them with the resources linked above regarding DNSEC errors with their nameservers. If I don’t hear from them after the weekend, I am wondering if I should just try to use some other nameservers from a 3rd party as an intermediate for adding the site to Cloudflare.