That experiment shows that whatever is stored on ProtonMail's servers plus your password is sufficient to decrypt your emails. This could be explained by the private key being derived from or encrypted with your password. ProtonMail's documentation says it's the latter (https://protonmail.com/support/knowledge-base/how-is-the-pri...):
> Your ProtonMail private key is generated in your browser. Before sending the private key to the server for storage, we encrypt it with your password (or mailbox password if you use two-password mode). This ensures that you and only you can use your private key.
Of course, there are other threats to worry about, such as ProtonMail changing their client-side JavaScript to exfiltrate your password. But the system as they've documented it does not appear to have any way to decrypt your email server-side short of guessing your password.
> Your ProtonMail private key is generated in your browser. Before sending the private key to the server for storage, we encrypt it with your password (or mailbox password if you use two-password mode). This ensures that you and only you can use your private key.
So the only remaining question is whether ProtonMail has access to your password. If they do, they can decrypt your private key and then decrypt your emails. Often, passwords are sent in plaintext to a server for authentication. But ProtonMail uses the Secure Remote Password (SRP) protocol so they never see your password: https://en.wikipedia.org/wiki/Secure_Remote_Password_protoco.... (source: https://protonmail.com/blog/encrypted_email_authentication/)
Of course, there are other threats to worry about, such as ProtonMail changing their client-side JavaScript to exfiltrate your password. But the system as they've documented it does not appear to have any way to decrypt your email server-side short of guessing your password.