If they don't know your username they can't offer a service. Hence zero knowledge services cannot exist based on your line of argumentation.

I feel you are being disingenuous though and you perfectly know what is commonly referred to as "zero knowledge", and specifically that the critical point is always "what does the zero knowledge applies to?".

Proton sees your IP, your login, email headers, for example. They don't know your email body and can't hand it over.