I think this is a good change for Proton. I hope it leads them to change some of their misleading privacy marketing.
That would be my guess.
The server still receives from other servers and from the client (if the email is not going to a proton account) the plain text, so nothing prevents them from making a copy.
They swiftly went on to win a court ruling that would prevent this specific situation from repeating. https://proton.me/blog/court-strengthens-email-privacy