That's right. The original Dulles terminal opened in 1962. There was no midfield terminal. For the ensuing twenty years it was EXTREMELY quiet out there -- think Mirabel, outside Montreal. The only operations were transcons that couldn't get into DCA and a few transatlantic flights. (Remember that in the '60s and early '70s all but az handful of northeast US transatlantic ops were out of JFK.)

Under these conditions all boardings were handled by the mobile lounges which left the Saarinen terminal and rolled up directly to the aircraft out on the apron. It was considered pretty space age at the time.

Dulles only heated up trafficwise in the early '80s when UA established its hub there and the midfield terminal was built to accommodate it. The midfield terminal was needed because Dulles was never conceived as a hub -- the original terminal was designed for origin/destination traffic only. Most folks who are just changing planes at IAD never see it, they stay out in the midfield. The lounges' main job has changed to shuttling O/D pax out to the midfield, but sometimes they do service planes directly as others have noted... I have never found that arrangement very convenient. When I lived in DC in the late '80s and used to take AA or UA redeyes in from LAX, we'd land IAD around 500am and the midfield terminal wouldn't be open yet, so we'd have to wait 30+ minutes to deplane for a mobile lounge to come get us... very annoying at the end of a long flight.