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.