More modern implementations of this use a time of flight check, so unless you have the ability to violate the laws of physics, boosting the signal so that a far away key transmits its signal to a nearby car is insufficient to unlock/start the car.

Are there many car keys actually using time of flight in the fob? Most of the cars I’ve owned use a much simpler approach - the key sleeps (stops broadcasting) unless moved. Drives me nuts with some fobs which have to be vigorously shaken to start broadcasting again and open my car etc. if key isn’t broadcasting, it can’t be mitm’d. It’s been awhile but I seem to recall time of flight being patent encumbered vs sleeping the key.

This obviously isn’t 100 percent full proof but likely works well enough for preventing many common mitm scenarios such as stealing from a car park or drive way most of the time etc.