Apple's statement can't really be independently verified
Can you independently verify unemployment? Customer satisfaction stats? The historical average temperature in Moscow, Russia during the month of August?
I personally think not.
That said:
I get what you're saying. I think it's not so much about independence as it is about lack of bias. And that's fair.
That said, when assigning "trust" to various information we see, we can rarely be fully confident.
Could Apple be lying? Yes.
Could it be more believable if they... released the exact numbers of subscribers/active users? Yes.
Could it be more believable if... they released the names and email addresses of every subscriber and active user? Sure.
You can see that as you assume more and more "access" to information, two things happen:
If you know these things, you might as well be working for Apple, in which case the skepticism targets you, as well.
My point is, this is what we can believe right now with the most confidence. Doesn't mean it's true in an absolute sense. And we practice this approximation of truth with 'trust' all the time.
If somebody wants to know whether you're happy, and you say yes, you could be lying. For that somebody to verify your happiness more, you would have to share with them things that are difficult or just inappropriate to share.
TL;DR: we often 'trust' information we can't directly verify to make the evaluation of our world more practical.
Apple could be lying, but the casual observer/gambler would place his bets on Apple's claims being true. Ultimately, that's the best we can do when choosing what to believe: the thing most probable to be true.