Not truly true...
Because in the Camera Roll "album" not only are there photos TAKEN with the phone's camera, but also photos SAVED from received messages, whatsapps, safari or twitter saved, etc...
That's why I've always thought that it would make a lot more sense if iOS stored the photos in two separate albums distinguishing between TAKEN and SAVED photos.
Normally, iCloud Photo Library, Photostream and all that stuff is intended to be used to have the beautiful pictures you've TAKEN with the device camera available to be viewed in all of your devices; but I don't think it was intended to sync all the funny photos, and all the stuff your friends or group chats' share with you...