While technically that sounds like a cool hack, it sounds like a dumb thing to actually allow to happen. "Okay Google" or "Hey Alexa" outside of the frequencies reproducible by human voices should be filtered out completely. At least, from my comfy chair nit picking someone else's work. Of course, by not filtering the acceptable bands allows them to do all of the ad tracking/rights management things they are allowing to occur. The fact that Alexa/Google is able to do these ad tracking/rights management is just another example showing that the mic is listening 100% of the time.

Last I looked, they used non-linear mixing in the analog circuity to down-convert ultrasonic audio into what looks like human speech to the devices.