Earlier quoted context omitted.
Yeah but messaging apps are really only useful if there are lots of people on them to message. So in the real world a relatively small number of providers, WhatsApp, Signal etc, are in a position where all your friends are going to be on them. And those are the ones likely to be named and told they need to implement image scanning/review.
Messaging protocols are useful even if everyone is not on the same app. In the past I was chatting with my google using friend via some third party jabber server where I had an account. It was useful and didn't require us to be "in the same app". We both were using both different apps and different server providers.
Exactly. That time is mostly over.