Well, yeah, nobody thinks they built it all in a single year. The SDK largely just publicized a subset of Apple's existing private APIs that they used to make their own apps. Foundation, UIKit, etc. — that stuff was already there, but you couldn't use it without jailbreaking your phone. Here's an article running down all the stuff that was included with the original iPhone, from almost a year before the SDK was announced: http://www.roughlydrafted.com/2007/07/16/iphone-os-x-archite...
Obviously, most apis themselves were done at the iphone launch, and wouldn't take much more work.
The rest, all the infrastructure needed to have a working app store would be a much more substantial project. It is this part that would be difficult to build in a single year, and would have had to be in the pipeline when Jobs made his statement.
Apple already had the iTunes Store and a lot of very mature associated infrastructure. The App Store was built on that, and a year sounds like a pretty generous estimate for the project.
