Do they get better prices than Samsung? Because Samsung is who designs and makes the flash memory. (Also, Wikipedia seems to say that Sony buys more than Apple.)
Fill in the blank.
Of that I'm a bit skeptical. For one thing, this sort of idea was fairly common about the G4 processors, even though Apple was by far the smallest purchaser of G4 processors -- Ericsson, Cisco, and Nortel were shipping routers using G4's in tens of thousands while Apple was still taking pre-orders in some cases.
Another thing to consider is that Samsung is among the world's largest semiconductor manufacturing companies. They fabricate and sell enough semiconductor products to make even Intel's volume look comparatively small. Even with the iVolumes that Apple produces, it's hard to imagine that even they could be a dominant consumer by volume.
The table on the Samsung page lists companies by fracton of Samsung's total sales, not just flash memory. Nor, for that matter, does Apple only buy flash memory from Samsung.