But if they use your input as training data, that would probably be enough.
My German isn't good enough to read the original text about this case, but if the sentiment behind https://natlawreview.com/article/data-mining-ai-systems-trai... is correct, I wouldn't be surprised if this would also fall under some kind of legal exception.
The biggest problem, of course, is that regardless of legality, this software will probably be used (and probably already is being used) because it's almost impossible to prove or disprove its use as a remote party.
That's something completely different. One is about copyright of stuff that was shared publically, while the other is about sharing private communications, violating their personal rights (not copyright).
But of course, we'll have to see, I'm not a lawyer either.
My family members all back up our conversations to Google Drive, I doubt WhatsApp would provide that feature if it were illegal.