> They may not even have a way to limit firmware to specific devices

They do. The new firmware may not be distributed, but it should be easy enough to make it such that if it was distributed, it wouldn't change security at all on phones other than the target.

For example:

    function handleWrongPassword() {
      if (deviceId = 1234) {
        return;
      }
      //do the normal phone stuff, like a delay or wipe
    }

Since the firmware is signed, no phone will run the firmware if you change the hard coded 1234.