So then you never use your email, right?

What?

Your email address cannot be (and isn't) secret, if you give it to other people (regular people, i.e. friends, colleagues, etc.) so they can send you emails. If you don't want your email harvested, you can never use it (at least to receive emails).

I think they’re obliquely referring to the scanning practices of major providers like Gmail, which most people use to filter their spam.

My experience is that you send email to someone whose Op Sec is not as good as your own, then your email will be harvested at the point when that person's address book is harvested. I don't know all the details of how these harvests occur, but using a shady mobile app with Contacts permission would be enough.