What's the allegation here? The mail stored on proton's servers is encrypted. If you send that mail elsewhere, it's subject to the security of the receiving server and any intermediary servers.
They are not denying that, they clearly stated it:
> ProtonMail does not voluntarily offer assistance. We only do so when ordered by a Swiss court or prosecutor, as we are obligated to follow the law in criminal cases.
That's not secret, or hidden by them.