That is completely not true. There is no way to make such a thing that can only work on one particular phone. There will be some point at which the compromised firmware image checks to see if it's that device, at which point it would be possible to change that to whatever device you want.
"This is the huge difference between this order (which I can live with) and blanket encryption backdoors using key escrow or other crap (which I'm absolutely vehemently against and willing to fight to the teeth)"
No, there is absolutely no difference between those two.