Actually the agent wasn't making this up. On the Alaska 737-800 there can't be a lap child in the very last row as there is no row across the aisle. There is a rule that there must be an oxygen mask available in every row for a flight attendant out in the aisle. This isn't a problem if there is an extra mask available on the other side of the aisle. But since that row only has seats on one side, then the extra mask must be available for the flight attendant. So no lap child in the last row. And, of course, never two lap children across the aisle from each other in the same row or there would be the same problem.