Perhaps I read it wrong, but since he was quoting a suggestion about assigned seating, I definitely read it as though he thought through-pax would present a problem when onboarding passengers who had paid to reserve the exit row or any '45-50' specific seats. I'm still pretty sure he was saying it would be a difficult task to make sure thru pax did not move into those reserved seats before the next round of boarding. If that isn't what he meant, my apologies - in that case, I'm the only one wondering what your solution to that difficulty would be.