I don't know a source for good public numbers on the app market, as most reports are very forward-looking. $25B for 2012 seems reasonable. However, Apple, who is a clear market leader, pulled in about $1.9 billion for all iTunes sales in the 3rd quarter, which is compatible with a $25B market. (Their take is 30% and they have about 70% of the app market, but apps aren't all of iTunes.)
There are various possibilities. It could be that most app developers (particularly independents) are satisfied with a far smaller income. Or, it could be that app developers are losing money as a whole (perhaps funded by VC money in anticipation of future growth.) Or, the 500K jobs number is fundamentally bogus, either inaccurate or double-counting.
For comparison, Zynga has ~3000 employees at about ~$300m quarterly revenue. $400K revenue/employee is in line with other tech companies. But if the industry is as successful as Zynga, on average, then that would be less than 100K jobs.