The problem here is that the “real” key fob is still the one in this attack doing the authentication, so it will still work regardless.
The problem is that this authentication happens automatically based on proximity — and the attack fakes the proximity, and not the authentication. The authentication here is still real, and doesn’t need to be faked — they’re not doing a MITM attack, just providing a bridge such that the car thinks the fob is nearby, at which point they authenticate as expected.
Your proposed solution doesn’t fix this problem, as it’s not an authentication problem in the first place. The attackers aren’t faking the authentication, nor are they even providing it — they just provide an RF bridge to boost the signals such that the car and key think they are in proximity, at which point the car and key authenticate and unlock the vehicle.
Hyundai does at least have an automatic power-kill switch built into their fobs when they’re at rest; however I don’t know if this is in use in the UK (where it appears the majority of attacks of this sort against the IONIQ 5 are made).
Yaz