iMac Intel 21.5" EMC 2544 Teardown
It's been confirmed that the Fusion Drive technology is software based and not the same as a Hybrid Drive. The two drives are separate, and act as one drive via software. Separating them allows for both SATA3 connectors to be used, and allows for more efficient bandwidth for your data, when it moves from one drive to the other. Also, if one portion of the Fusion Drive fails, it can be replaced. So iFixit is correct in their assumptions.