Remembering the crystal radio I built as a kid, this seems to be a novel approach using a new technology that has only been around for a bit over a century (not me, just the radios). They really use no external current source other than the radio waves.
The academic research I presented and remarked upon at the 2015 MSIG MEMS Executive Congress consumes zero power in standby. As in really zero. The Vesper product you feature here is *not* zero power in listen mode - it is ultra low power.
A 3uA quiescent current will still eventually drain a battery. A true zero power device will never do so. That's the significant difference.
While a 3uA quiescent current may be news for MEMS microphones, it is old news for MEMS accelerometers, which have had a low power standby-mode feature for some time. Check out the ST LIS3DH 3-axis accel. Its ultra low power standby mode consumes only 2uA.