Apparently it was supposed to be. They have the core functions in a "can't OTA" container, with less important functions like AV/etc in a "can OTA" model.
This update was pushed to the "can update" side and fucked the "can't update" side, which is its own can of worms. Another can? Jeep pushed a silent, emergency update "to all Jeeps even those who have automatic updates disabled."
So the issue is not that they have no security model, it's that it clearly doesn't work and they can and will push secret updates even when you decline all updates.