Verification is very simple: if you log in on a new device and see all your content while using only login and password to authenticate yourself, then the content stored on a server is NOT encrypted and is readable by server owner.
What about if the encryption key is derived from your password? This is common enough for "encrypt file with a password" services, I've personally implemented it in-browser as part of a small project.
Now, having your account password be the same as the email decryption password is also probably a bad idea, but we're far from the server owner being able to read your emails.
Protonmail keeps generated public and private keys on their servers.
Protonmail do not see your password and without it cannot decrypt the pub/private key pair.