Hmmm... 10 Billion dollars in app revenue divided by 627k is only about $16k/annum. That's around or below minimum wage in most U.S. states. Maybe that job number includes "trickle down" from developers who make a lot more, but spend their app income at business that use the revenue to hire/pay more gardeners, grocery baggers, fast foot clerks, etc.
Or maybe the number includes a lot of part-time hobbyists who would love to put a few extra grand into their bank account, or just buy a new MacBook, strictly from app revenue over and above the $99/annum fee.
All I know is that, as a mobile app developer with over 18 years of experience, there are vastly more people working on iOS app stuff than there ever were working on PalmOS, Magic Cap or Newton apps. Not only coders, but UX designers, artists, composers, makers, marketing execs, back-end IT support, and etc.