I think you may be missing his real point: he’s asking people to stop sending anonymous non-attributed emails, as he deletes those.
I do the same; it’s not a proton/gmail/etc. thing, it’s a “did you say who you are in the mail” thing. He then points out that most of these anonymous messages are coming from protonmail/gmail accounts.
He’s essentially saying he doesn’t want anonymous tips/feedback. Nothing to do with the delivery service.