Well the logic behind that is; it's far more likely that a business traveler, as opposed to a family, would be making an airport connection. The reason for this is it can get rather expensive for a family of 4 to take the train to the plane.
On the other hand a business traveler has an expense account, plus they are most likely going downtown into a city. So they won't need a rental car if they can take a train. A business person however, would be in a bigger hurry than would a family, so they'd want a faster train.
Plus in the case of Newark & Newark Airport, there are 3 times as many hotels at the airport than there are in Newark. So a business traveler would be more inclinded to stay there as opposed to Newark. Yes, if they are headed for NYC, then they will probably stay there.
On the other hand if I lived in DC, and I needed to come up to NYC for work, I might well take an Express to EWR. Then ride NJT into the city each day. The reason for this is hotels are far more expensive in NYC than they are at EWR.
So it's kind of debatable, as to what's best. Frankly the best answer would be as follows. First, get Amtrak the money it needs to get the bulk of the WAS-NYC corridor up to a 150mph speed limit. Second, alternate the Acela Express trains to stop at Metro Park this hour, then EWR next hour. This would provide service to everyone, and at 150mph the ride would still be 20 minutes shorter from NYC to WAS.