Yes, I simply don’t buy that either. I try comparatively hard to use Siri, and I don’t think I write more than 5% via Siri. It doesn’t work that well even while driving.
I would guess that this statistic is about using the voice button on the keyboard, rather then commanding Siri "send a message to...". I've watched people that do this constantly, and perhaps they send more messages overall as a result of the behavior.