One thing is definitely true: a year ago the phrase, "...but it's not really a tablet market, it's an iPad market" was so often used as to become annoying. I haven't heard that phrase in a few months.
First, when the iPod appeared, the whole ecosystem wasn't really about the "OS" of the device, and now iOS has a real competitor in Android, that a lot of other manufacturers can use, while back then there was no such unification from the other manufacturers. This matters because the iPod was a single-task product, while tablets are not.
Second, they said that Apple has its own stores, while the others do not. But we're living in a global market now, and Apple's stores are virtually non-existent outside of US. Having 1 or 5 stores in a whole country doesn't make much of a difference. So even if Android tablets completely fail in US, there's a huge market out there for them, outside of US, which is bound to become bigger than the whole "iPad market".
Such is not the case with apps. If you've spent a few hundred dollars on apps over the course of a few years using iOS devices, the cost to switch (assuming you care about continuing to use any of those apps once you're on Android) is pretty high.