The article is not claiming any lockdown command by AT&T. It says that the phone locks down when it can't get a "mandatory" update. And it won't get the update

The article is not claiming any lockdown command by AT&T. It says that the phone locks down when it can't get a "mandatory" update. And it won't get the update if it's not on the "right" carrier. At least it seems possible. If Alpha is right we will know soon.