They have clarified the situation: “You could always report messages from any group to moderators, acts like forwarding. Private chats are still private too – although you could always report a new incoming chat to moderators by using Block > Report. Anyone can check our code and see there were no changes. The FAQ change made it clearer how to report things, including via DSA.”
As a user for almost half a decade I can confirm this button has always been there, and I have witnessed multiple private groups getting banned upon reporting
Yep, nothing new here.