Very impressive but use case is narrow. You are on a train and don't have time to make a note, ok. But most of the time we are in places reasonably private where speech recognition is fine and would be convenient maybe more so learning to say without saying.

As a disability speech aid though maybe it would be amazing?

Speech for the disabled would indeed be great, as long as the disability doesn't also affect the system which you use to "silent speak".

As for the privacy thing, I would say that I absolutely hate talking out loud to my devices. Just the idea of talking my ideas into a recorder in my own office where nobody can hear me feels very strange to me. But I love thinking through ideas and writing scripts for speeches or presentations in my mind, or to plan out some code or overall project. A device like this would allow me to do the internal monologue thing, then turn to "silent speak" them into this device to take notes which sounds great. And the form-factor doesn't look that dissimilar to a set a bone-conduction headsets which would be perfect for privacy-aware feedback while allowing you to take in your surroundings.

With this tech demo though it seems like the transmission rate is veeery slow, he sits still in his chair staring into the room and a short sentence is all that appears. Not exactly speed of thought..

And of course there is the cable running off to who knows what kind of computational resources.

The AI parts of this are less exciting to me, but as an input device I'm really on-board with the idea.

There are times when I'm masked (ie: 3M respirator) and gloved at work for 10+ hours at a time. I wonder if speech recognition would be possible without breaking the face seal during silent speech. This could be beneficial for other types of work in hazardous environments.

(My current solution is to tear the fingertip off my offhand glove so I can unlock and use my device....)