I mean, he's smart enough to not do this by accident - if he wanted people to speak in a "medieval" fashion, he'd make them do so. I'd hazard a guess that by having them use modern colloquialisms, he wants to reduce the gap between the reader and his characters - illustrate that everything is "modern" in its own time, and that the mindset of people in a medieval or pseudo-medieval society will in at least some regards be quite similar to that of 21st century readers. Fantasy often tries to increase the gap between readers and characters, to enhance the escapism or just because readers find it somehow more convincing when the characters speak in "medieval" ways, whatever that may be. But it seems like a fine idea to me to do the opposite to make his characters seem more real, and his world more original - not just some generic fantasy universe at an easily determined "tech level" that just happens to correspond to some particular period of real world history for the reader's convenience.