How do they know he wasn’t exhibiting symptoms, albeit mild ones?
Were they sitting beside him monitoring him?
So even assuming he wasn’t feverish when he boarded the flight in Africa (which is a big assumption), he had a long journey. He could easily have been showing symptoms on arrival.
Idiocy.
I’ve got a better one.
IF he stayed in DC any amount of time and then re-entered the airport and got rechecked by security not ONLY could he have been early symptomatic and spreading it via stuff he touched that’s not all. His baggage that he touched could have been checked. And jostled against who knows who’s baggage in the handling facility. If wiped a big snotty booger off his nose and then put his bag on the scales and thence on the conveyor, just having a bag go through Dulles that day or maybe the next could be a risk.