In the story, they implemented table (building) and row (floor) level permissions simultaneously. So you had to swipe into the building, then in the elevator to get the elevator to stop at your floor.

I guess I could see contention possibly happening as described if everybody arrived almost simultaneously and both swiping points had very high latency. But why not keep the door checkpoints armed and disable the elevator swipes? That makes me think it's a contrived example.