what kind of consumer level antenna can forward/amplify key fobs (in the gigahertz range, no?) without causing excess “signal to noise” ratio that the car can detect?
A obvious countermeasure for such attacks would be to have the car measure the RTT between the car and fob, exchanging some cryptographic credential. If it takes too long the fob is too far away and/or an attackers repeater is adding delay.
For the purposes of a "relay attack" this doesn't actually matter: all else being equal you could devise a relay system that works regardless of the roles of car and fob in the protocol.