Some notes, on why I think your calculation while interesting, is way off:

1. You shouldn't consider summer 2008 as epoch.

2. Not all of those $4B were paid to US developers (I'd guess less than 60% of that)

3. A huge number of apps (thousands) on the App Store are free (from Google, Facebook, Amazon, Zynga, ...) - For all we know, all such free apps have been created by a team of 10-15 iOS developers/testers/etc.

I'm also skeptical about the claim, but I think that your calculation wasn't a good one.