What GP likely means is that a Protonmail user can exchange encrypted emails with a non-Protonmail user without doing anything except importing the non-Protonmail user's keys once.

This could allow a single technical user with an elaborate PGP key and email setup to convince his or her social circle (or at least email circle) to switch to Protonmail and follow a set of (comparatively) simple steps to get Proton to recognize the key to their non-Proton address.

Thereby having PGP-encrypted conversations with users who don't know how PGP works, but followed their friends' instructions to use a slightly different webmail interface. Meanwhile the technical user can use a smartcard, a self-hosted email server, or other madness that a normal user would never tolerate.