But of course Apple has tons and tons of non-Apple hardware running OS X. In order to market a computer, you must first design and test it; and, therefore, Apple will have plenty of prototype devices of all sorts (assuming that they haven't just stopped doing Mac R&D work).
I would guess they still have plenty of Xserve-like devices lying around, or older Mac Pro enclosures containing more up-to-date hardware (given how much easier it is to install hardware into the classic Mac Pro than the cylindrical Mac Pro). And yeah, probably even devices from other manufacturers...