"null record" is not "no record". It's an MX record of priority "0" and value "."

No MTA will try to deliver a message that is from or to a domain that has a null record. If you're the sender, your sending MTA might give you an NDR, but receiving infrastructure will just drop it.

Cloudflare actually has a wizard that implements it in addition to the items on the page you linked. And no offense to whoever wrote the CF article, but they are the new kids on the block relative to email.