Client-Side-Scanning is going to be implemented by the messaging app vendor. This means that it’s limited by OS or Browser sandboxing . Therefore it’s definitely limited to what the messaging app has access to. However, I’m not sure what the actual scope would be, meaning if all accessible images are going to be scanned or only the one being transmitted to someone.
Good for you commenting on the title alone! If you looked at the actual article you would know that one of the limitations is, that the furthest point you can reach going back in time is when the “time machine” was first activated.