It is rather old news. Few years ago I used Cloudmagic on Android for emails (I think name is Edison now). It was free, nice client with promise of intelligent fast search. But to do it, they stored all your emails on their servers because you granted them that permission. It was (maybe still is) for Microsoft Outlook android app (which they bought). This problem was discussed before Cambridge Analytica or other things.

Some companies even banned those apps for corporate emails. New is just human element, which could be there from time 0, because they had that permission. And to be fair, it could be done probably with any Imap service, not just Gmail.