If most people you email with use Gmail, what's the point of an "encrypted email" provider? Why not just use PGP and stay with your current provider? Thunderbird+enigmail is the way to do meaningful encryption and it is provider agnostic.
I have no idea, I'm not the one advertising the feature lol. I literally said that I've not even researched them, and that it is "perhaps end to 2 end". Beyond that, I know nothing about them.