Hmmmm. Alexa hears some ultrasound.

Actually, all the voice assistants do. Siri, ironically, is the most secure as it requires an exact voice match, but they are all susceptible. This is why I tell people all the time to NEVER keep a voice assistant near their computer because the range of a ultrasound command that is able to trigger a voice assistant (Alexa, GH, GA, etc) is about 1 meter max. I also keep the begin/end request sounds on on all my voice assistant devices so that I can hear when any of them are triggered.