I don't think the water has penetrated the shaft that quickly to cause any damage. If you put the iron on too high of a temp that will cause bubbles to form and shaft damage. But, that is with any iron. Remember water boils at 100C. So the water inside the hair shaft would boil and cause shaft damage.
ETA: I think that all flat irons cause damage. Especially if the iron is too hot. Essentially if you are boiling the hair from within everytime you flat iron if the iron goes above 100C or 212F. I think that is why it is essential to your hair health to use heat protectants.
I'm not a flat iron experiment far from it. But, with the bit of research I have done, before purchasing a flat iron for my natural hair, this is what I have determined thus far.