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Neat idea but how would it protect against a relaying/signal boost attack?

Not sure if they are, but it could be done by having the device measure the round trip time of the signal, and refusing to answer if it's too long.

Most likely not because the turn around time in a challenge and response, and clock skews between devices is far greater than the range boost.

Maybe it has a button on it?

I hope so. Their whole angle seems to be 'automatic access via proximity'