If you look at his facial movements in the video it looks as if he is pretty actively using his facial muscles, 'trying' to speak while moving as little as possible (which would cause the clearest signals to be emitted).

If that is what is happening, to me it feels like harder work than just speaking (similar to how singing softly but accurately can be very hard work). It would still be pretty cool, but only practical in use cases where you have to be silent and only for short periods of usage.