The vast majority of senders do not send email using end to end encryption. If you’re sending an email from a PM address to another PM address, sure, it’s end to end encrypted. If you’re sending to another service, it’s not end to end encrypted unless you’ve both gone through the painful steps of setting up PGP encryption. Same as if you’re receiving from another service.

You can read about it here:

https://proton.me/support/proton-mail-encryption-explained

So that quote you just responded with is saying exactly what I had just said above it. They promise that they’ll encrypt that unencrypted email that just came into their server for you. And they promise that they’ll encrypt that unencrypted email you just sent outside their service.