Unless you use end-to-end encryption (like PGP) then that's not true.

I have a protonmail account. When I log in to the interface, I see the body of emails, without providing any key on the client (not that it'd help since the client is a generic browser running their website code). This implies the process exists to recover the body of my emails. Also, I type in the password in their web UI in cleartext - there's no other way to gain access - which means they also have access to my cleartext password and could be forced to disclose it to the third parties. So unless you provide some contrary evidence, your assertion is false.