Curious how much intentionality is required from the user to produce sounds. It would be unfortunate if this device just started firing off speech for what would otherwise be thoughts one would not say out loud. I suppose that depends on the mechanism required to activate the neurons to which the device is connected.
Even without this device there's been some consideration to the thought that the conscious brain is merely an observer since it appears to activate after the unconscious brain takes actions. You just go along with what the unconscious mind did in actions and speech and you convince yourself you meant to do that after the fact.
https://pmc.ncbi.nlm.nih.gov/articles/PMC3746176/
So here it could indeed just fire off speech and you know what? We'd probably convince ourselves that we absolutely meant to do that. In fact it could be a very interesting experiment (with willing participants). Mess with the inputs the device receives so it's not really the person activating it, let it do it's thing and see if they notice when they do/don't have control of it.
Without having RTFA, I'd guess/predict that it will be possible to learn to only do this intentionally, much like we can think about raising our arm without actually raising it.
I hope it's something better than "Hey Siri, say..."
Otherwise, yeah, that would be a new sort of hell where you had no private inner monologue
The brain is incredibly adaptive, I guarantee eventually it would learn to avoid firing the neurons the device is probing when you don't want your inner monologue spoken aloud as long as there's a feedback loop where you experience negative emotions when something you didn't want spoken aloud was broadcast.