Non PGP encrypted mail (99.9%) is only encrypted via transit by TLS V1.3 at best. Proton has no choice but to decrypt your mail as they recieve it, then Re-encrypt with your PGP key secured by your proton password. They can see all your mail, as a fault of the insecurity of the SMTP protocol. It's only Zero knowledge after they re-encrypt, you have to trust them not to scan 100% before they encrypt.