I love how this article is speculating on what crazy things Amazon must be building by adding that many people. I'll tell you what they're building: fulfillment centers and call centers.
About 5 years ago when Amazon had around 15K employees worldwide and they only had 3K in Seattle and only a few small dev centers around the world. So even if every single person in Seattle was working on development that still only accounts for 20% of the workforce.
I can't find up to date numbers on the number of people in each location but considering there's now 40 FCs worldwide I don't think my math can be that far off. (http://en.wikipedia.org/wiki/Amazon.com#Locations)
If I had to bet, I'd say 2000 of 11000 new employees are either developers, dev managers, or project managers.
It might also be worth asking when the headcount for Zappos got tacked onto Amazon.com.