Unless it was in my pocket and I was walking around with it, which I do every day.

True, the solution isn't perfect. Just brainstorming ideas that can potentially be coupled with other layers of security. The advantage of the idea is it doesn't rely on the user activating a secure mode or knowing about the vulnerability. The protection is automatic when you're asleep. Arguably if you're walking, not a large amount of that time is near your front door - although I understand your location may still be predictable.

Why would a motion sensor not sense motion in your pocket?

I think they're saying the thieves could still execute the signal boosting attack while the car owner is walking around with their keys.

The parent is rightly pointing out that it could yield an unintended RF activation.