Parent is suggesting that would be an effective screen for people with other symptoms. Hence, "If that screen loses its effectiveness, you start to have to worry about every patient presenting with a fever and diarrhea..."

So at the moment, this "screen" is an easy way to rule out Ebola in a patient with potentially Ebola-like symptoms. Obviously we wouldn't expect everyone who has recently been to Africa has Ebola. I didn't downvote you, but I expect this is the reason why you're getting downvotes, not a misunderstanding about your question.