It is not that clear cut
From the article:
"But that's the point: the customers are the app developers' customers, not really Apple's or Google's."
Well, yes. And no. When I buy, say, Glenfiddich whisky from Tesco am I Glenfiddich's customer or Tesco's? Obviously both. Separately, and jointly, and my level of customershipness may be different from the next person who buys the same product from the same store.
You see, I like Glenfiddich, and I chose to shop in Tesco. If I have any problems with the product I would go back to Tesco for resolution in the first instance - and only Glenfiddich directly if I got nowhere with Tesco (or if the problem was so serious it needed escalating also), however I would not hold Tesco responsible for product quality issues, just like I would not stop buying Glenfiddich if a Tesco staff member was rude to me.
But ultimately, do I shop in Tesco because I can get my choice of malty goodness there, or do I chose to shop in Tesco which means I select my heavenly taste of Scotland from the selection Tesco elect to provide? You can't tell from the simple sales figures - and the same with apps. I have an iPhone with many iPhone apps - but you can't tell from that statement whether I chose an iPhone because I wanted an iPhone (and therefore am stuck with the pool of apps His Jobness permits me to select from) or whether I looked at the apps I either wanted or had already paid for and was forced to buy an iPhone on that basis.
Arguably, back to the apps point, I am a customer of Apple, and Apple are a customer of the app developer who developed the app I chose to buy - I pay Apple and then Apple pay the developer after all.
We are paying Apple to provide a service - they make the apps available to us in a way we want and Apple perform a number of checks and balances to improve the quality of apps* available - this makes Apple more than just middle-men and we are happy to pay for similar services in other areas: we pay for shops to make it easier and safer to buy goods, we pay for travel agents to sort our holidays, we pay our governments to educate our children, we pay record companies to give us quality controlled, professionally produced music on shiny spinny-disky-things, we pay estate agents to forcibly make non-consensual love to our bottys - sorry, to facilitate the buying and selling of our houses, and so on. So why is paying an app store that much different?
*by quality I mean in the "won't brick your phone" way, not the "actually any good" way.