They can't change the terms of the warranty, but it can expire which it probably has on a 2013. A warranty is a contract, one party can't arbitrarily change the terms of a contract after it's been signed, if that were possible then contracts would be meaningless.I don't believe it's right to change the rules in the middle of the game such as this, but the EVSE warranty has been a huge topic of confusion with some dealers stating that yes it is part of the Voltec warranty since the EVSE specifically states "Voltec" on the device, and a vast majority of others stating it's only covered under the B2B warranty.
That said, I have not seen anything in my included books on the warranty coverage for my '13 that specifically put the EVSE into the Voltec warranty coverage. The B2B coverage pretty much covers everything, so denying coverage would need a d*** good excuse from GM, in this case however they could state it was a case of abuse and deny the coverage.
Finding a replacement EVSE can be done for about the same or less than what one would pay GM for an OEM replacement. On the other hand, if you only broke some of the plastic for the gun type plug that goes into the charge port on the car and your reasonably mechanical then you should be able to easily replace the J1772 plug. They're usually about $80 or so which makes them not exactly cheap. On the other hand, you can find an EVSE that can do both L1 and L2 for about $100-150 more. It's just a matter of plug type and input voltage since they're auto switching and take 120-240V.