The act of coding itself is also not a constant flow of characters from fingers to screen. You stop to take a moment, go back, correct something, next line, pause to think, type out a line etc. I have never used a device like this but I'm pretty sure I type at roughly the speed I think, at least when I'm writing programs. General note taking, eg. jotting down ideas, might be different, where I do find myself thinking faster than I type.