The GovCloud AWS region is located in the Northwestern USA, so it seems more likely that if he was doing GovCloud specific work, they'd have hired him in Seattle.
Though I'm sure AWS does lots of work with US government agencies in us-east-1, so it's not surprising that a government clearance might be required for some of it.
GovCloud is indeed in the Pacific Northwest, according to the 2011 announcement from Amazon itself. Specifically, it's in Boardman, Oregon, on a road named after famous explorers. It was their first structure in the area, confirmed by aerial photography and contemporary reporting. Smaller modular backups were active on a different site in Boardman right next to the interstate, and outside Umatilla near the prison. These other sites eventually evolved into the rest of us-east-2.
As for us-east-1, it all began in Chantilly in an existing low-rise building you could mistake for rentable office space, then expanded to several different sites between Sterling, Ashburn, and the airport itself, and perhaps surprisingly, to another place where famous battles were fought. Finally, they have a site a bit further away, near where Disney was going to build a theme park. That's where they want to expand more, but first they have to convince the power company to get them a dedicated line, instead of being fed from the same place as the battlefield.
Though I'm sure AWS does lots of work with US government agencies in us-east-1, so it's not surprising that a government clearance might be required for some of it.