I think they talk more about encryption of the e-mails during transport. Keeping e-mails encrypted on a server is doable and not very hard (Protonmail does this

I think they talk more about encryption of the e-mails during transport. Keeping e-mails encrypted on a server is doable and not very hard (Protonmail does this for exaple), but encrypting e-mails while they're in transit and go through some possibly untrusted mailservers is more challenging, because it usually requires asymmetric encryption e.g. via PGP. This in turn requires that the sender has access to a valid and trustworthy PGP key for all recipients of the e-mail. Distribution of these keys must be based on some form of trust, which in itself can be challenging to establish (hence services like Keybase). Also, while good solutions exist for using PGP in web- or desktop mail clients, usability is still not very good especially for people that don't have a technical background (in my opinion). I use PGP-based e-mail encryption a lot at work and I can say that there are still a lot of things to improve here.