No it's not. You just publish another TXT record.

...... and sign all outgoing emails.

Which is not hard to do. It's literally a switch in O365. It takes a bit more in other environments, but nothing a competent admin should really struggle with.

so your opinion is that anyone who self-hosts their email is not competent? You know, there's a huge amount of machinery behind that toggle switch Microsoft offers you.

No. I think that people who can't figure out how to install openDKIM and connect it to Postfix are not competent email admins.