This is where the confusion lies because it hasn't been clearly communicated by the FIA - likely because they had no idea what they were doing.
I don't think anyone has ever actually said: Martin Prokop, Solberg, Bertelli etc - you can't drive a 2017 WRC car. That's just something that has been assumed due to lack of proper regs or information. Really I think the FIA just don't want some young teenager or some old dentist to be able to drive a 2017 car. They've made a rule that they don't really understand or know how to enforce. Creating a problem where there wasn't one.