If the conductor is listed and labeled for a specific maximum voltage then it shall match the voltage of the circuit.
" . . . to which it is connected." The EGC isn't connected to a circuit, and its operating voltage is 0 volts. During a fault conditions the voltage will be higher, but those are not operating conditions.
I have yet to see any text in the NEC which says that a wire which is allowed to be bare, if it is instead insulated, needs to have a particular minimum voltage rating on that insulation.
Edit: The wording of the second sentence of 300.3(C)(1) is slightly ambiguous, but in context the word "conductors" there should be read as "circuit conductors" as discussed in the first sentence. Otherwise it would, for example, imply that bare EGCs are prohibited in an enclosure containing circuit conductors of different systems.
Cheers, Wayne