Yes I simplified. Some(? I’d hope all but probably not) new fobs do turn off (ignore the car broadcast) if they are not moved for a time. I proved this to myself with my 2020 car by putting my keys down by my car door, I could only unlock the car for a minute or two after I put it down, after that keyless entry didn’t work until I disturbed the fob to wake it up.

This is to mitigate the relay attack at home (and I’m sure other times, like if the key is in a purse), one avenue was that attackers would count on people hanging their keys by the door, so accessible to selective standing on the stoop with a relay. By turning off at rest they can’t be exploited this way.

Older fobs never turned off - so they are constantly broadcasting the signal for the car. Newer fobs do turn off when at rest so they’re less risky, but if say it’s in your pocket it’s constantly moving so someone could still relay it to steal your vehicle, assuming they get close enough to you.

The faraday bag is good for older fobs or if you think you’re at risk of a key relay attack.