No I am not referring to domain registration. I am fairly certain that if I want to use their free accounts I would have to point the root servers at them to manage my zones even though they are registered through a dozen other registrars. In other words I can not manage my own DNS if I use their free accounts. It's not a big deal since I also do not use them. I make my own tiny CDN's when I need one. It's all hobby sites for me these days, retired from tech and no longer manage big zones.

So in other words, instead of being able to have the root servers provide the IP addresses of my bare metal servers running NSD NS records I would have to tell the root servers via the registrars I use to give the NS names/IP's of cloudflares DNS servers. The domains are still on the dozen registrars I use but CF have to be authoritative for them for the free accounts or at least that is they way it was when I first played with CF after they stopped being honeypots that I contributed to. I would say custom DNS but nowadays that means 50 different things to 50 different DNS admins on HN. It's just apex NS records in the root anycast clusters.