having looked at the data
I can conclude that the worst thing you can do as far as reaction times are concerned is to drive an actual car. What you need to do is drive only in a driving simulator.
This test seems to ignore the most important difference between hands free and hands on systems. That being that in one system, one or all of your hands is tied up controlling the device whereas with hands free, both hands are available to control the car. This is particularly important in countries like the UK where the majority of cars have manual gear boxes. I don't think anyone is arguing that your reaction times for talking on a hands free phone or directly into the handset are going to be similar, that's just common sense.
I don't use the speech to text on my phone, it only works properly when the car is stationary so there is no background noise. I do, however make use of the text to speech a lot.