You are quite right. I mistakenly added the 1.2 million to the 600,000 and the nearly 100,000 on OSX. This is good news. Nearer $10,000 per app per year isn't disgraceful as an average. Wasn't the average much less a couple of years ago, like under $1000 per app?
Not the calc I would use, but If I divide 5,000,000,000 by 1,200,000 I get approx 4000 per app, BUT that would assume that they were all paid for last year which is not the case, and as others have mentioned, there would be a massive amount of paid apps and apps that are never purchased or have very few purchases.
You could look at the total amount $20,000,000,000 / 1,200,000 and get an average per app of $16,667 per app but that is pretty useless too (free apps, apps on OSX costing more, most apps downloaded few times etc and also not taking into account the apps that are no longer on the app store)
The post doesn't give enough info to work out what individual developers are likely to get.