For clarity, the author doesn't like unencrypted but signed emails. Which strikes me as weird but at least it isn't something that people would normally do. They are OK with encrypted and signed emails and presumably encrypted but not signed emails. Here is the rationale:

* https://k9mail.app/2016/11/24/OpenPGP-Considerations-Part-I....

I think there are implementations out there that don't let you send encrypted but unsigned messages (anonymous encrypted). This stuff seems to bring out the enforced opinions in people.