It's just a weirdly worded recommendation. A few years back, Cloudflare and Amazon did DNS but didn't sell domains or didn't sell your TLD or to your region. A

It's just a weirdly worded recommendation. A few years back, Cloudflare and Amazon did DNS but didn't sell domains or didn't sell your TLD or to your region. And in general, places that were regarded as reasonably good at selling domains didn't have anything in the way of an enterprise grade DNS service. You would go to Namecheap or Porkbun or something and get a reasonable domain they had nothing like the functionality you'd get by putting DNS somewhere better. It was well established for a long time as a fact you don't mix these roles, it's only recently that decent players started doing both.