Here’s how I understand the issue:
A keyfob is a radio Transmitter. To unlock your car you need the radio transmission to reach the car. The keyfob doesn’t transmit a signal when at rest. Therefore putting a keyfob in a Faraday bag achieves nothing.
… Which means that if the hacker is near you when you park - there is a time period where the fob isn’t masked by the bag, because it is coming out of the ignition, and voulaa - you can record the key’s pong of the car’s ping, retransmit, and get in. Correct?