Are you sure apple has no way of tracking how many songs are being streamed over iOS? I'm not sure (either way). Seems it would be pretty easy and legal thing for them to do so long as data was kept pretty anonymous.
I don't know, not do I claim to know, any of the answers. I am simply musing here. I still don't think it's far fetched to make the comparison of iPhone having the most music played. Other players which sell a fraction of total sales compared to iPhone would have to have a much higher percentage of those sales playing than the iPhone counterpart, as I said before. This could be a claim that Apple made with some simple deductive reasoning.
Here is an example. For simplicity's sake, let's say iPhone sells 100 units versus 50 units on Android X. A mere 25% of iPhone users enjoy music, meaning 25 units. For Android X to match that, a whopping 50% of sales (or double the percentage of iPhone sales) would have to enjoy music just to break even with the iPhone numbers. iPhone sells so many more units, at least in the states, than any other single phone that it's pretty easy to come to this conclusion without any other data.
Lastly, I will say that I don't know that broad claims like this need to really be backed up. In the Mac v pc ad days there was a router that was supposedly was plug and play with the max but not pc. Have you ever met a router that's not plug and play on a pc? I absolutely have no, ever. I was actually quite upset by those claims in those days as I was a die hard pc guy at the time.