It sounds like a genuine attack vector to me. If someone hacked say teslas firmware supply chain and made all chargers pull max power at the same time, it could be a national infrastructure crippling attack.
Where would they dump the power? You need a load of some sort. 7kW requires a voltage drop between a hot and a neutral. If it's a 1V drop then you're going to get a hot load of amps.
Im thinking in an attack situation hackers might plan for say a 3am mass dump and pull on the grid by all cars that are charging overnight. This would definitely be possible by altering firmware, and would be bad enough to blow some local substations for sure.