It was originally intended for "expert" flyers, yes. And there were signs as you approached it defining what that meant. But there was never any enforcement - anyone with a boading pass could use it from the first day it opened.

But it was never used for "crew". Crew and airport employees always had a separate security checkpoint - that checkpoint is currently between baggage carousels 7 & 8. What you might be remembering is that the locations for Dulles Diamond checkpoint and the crew/employee checkpoint were originally the opposite of the current setup. For some reason, their locations were swapped over a weekend a couple of years ago.