Zero-access encryption
Your chats are stored using our battle-tested zero-access encryption, so even we can’t read them, similar to other Proton services such as Proton Mail, Proton Drive, and Proton Pass.
from protons own website.
And why this is not true is explained in the article from the main post as well as easily figured out with a little common sense (AI can't respond to messages it can't understand, so the AI must decrypt them).
Time for Mullvad VPN?