150-200’ of separation on short final is a near miss in any book. And that was designed into the system. Altimeters are considered acceptable for IFR if they are accurate to within 75’. If we give the raw data of every flight for a month through that airspace to a statistician and they could generate the odds of two aircraft passing the same place in space at the same time. Assuming everyone does everything correctly, I would bet the odds are NOT zero. Also, visual separation is probably much more reliable in the daylight hours in a tight corridor like that DCA airspace.