I don’t think it’s the case even on a paid tier, if it is I can’t find any reference to it. There are a lot of posts on their community support form where the recommendation is to use another registrar. https://community.cloudflare.com/t/use-cloudflare-registrar-...

I do see an option for custom nameservers on my dashboard, followed by a button "upgrade to business".

That option means "your nameservers are now ns1/ns2 on your domain but they still map to Cloudflare", not "you get to pick your own third-party nameservers".

I see, thanks.