The post would be better if it clarified what threat model they have in mind. For tampering, PGP-signed email is helpful. For protecting secret communication

The post would be better if it clarified what threat model they have in mind. For tampering, PGP-signed email is helpful. For protecting secret communication contents, PGP-encrypted email can be helpful to some degree. For preventing a surveillance from knowing who you talk to, when, from where and what type of content you likely exchange, email cannot become secure. I suspect it is the combination of the three the author has in mind.