I’ve seen a few articles about this (one called it “mind reading”) and I still don’t understand how it works. What exactly is “vocalizing internally”?
I guess the device was first build to recognize actually spoken speech with the signal that is transmitted to the muscles. They gradually reduced the speech levels, and now the system is efficient even with tiny signals.
Producing speech is an incredibly complex task; taking most humans more than 2 years to acquire. Loads of different muscles are involved. I would not be surprised if the different signals they collect in their device are more than redundant for the task.