“This requires a powered USB hub since Mac Pro offers four USB ports and you will need six to connect them all.”
This has nothing to do with USB ports; there are six thunderbolt ports on a MacPro 2013. For six DVI monitors, 2 of these monitors have have passive TB->DVI adapters, the other 4 adapters must be active. The MP’s HDMI connector counts as though it were a passive device.