Proton can be legally ordered to start recording the IP address of a specific user. That’s why they recommend that you always connect through their Onion site.
Other than that and if that’s possible, I think it may also be possible to legally order Proton to keep the unencrypted form of incoming emails for a specific user, but Proton did not said it in the article, and Swiss laws might protect them against that. It’s certainly possible technically, and good to be aware of it, I think.
Sorry but I can’t open the second link, as it actively resists it. I suspect though that the problem with Tutanota was not their encryption, but their legal system, which required them to keep a copy of the incoming emails.
Also, don’t mistake me, I’m all for protonmail, and I mean this. But did you know they only encrypt the email contents? Metadata like title, sender recipient and other things in the mail header don’t get encrypted.