what about this is usb 3? the only thing this does is charge usb devices while the imac or thunderbolt display is OFF.
It is described as a USB 3 *hub* so, presumably, like other USB3 hubs, it has a USB B-to-USB A cable (or a captive USB3 A cable) that goes to one of the USB3 ports on the iMac, turning 1 USB 3 port into 4.
That's a guess - the announcement is an information-free zone.
Hopefully, the USB-in cable is located under the power cable on the bit that slots into the iMac stand, but they haven't cluttered up their nice computer rendering of the product with any such detail, so that's still a guess.
Of course, it won't be USB3 if you use it on an older iMac or a Thunderbolt/Cinema display.
Personally, I think what the iMac/Display needs is not more ports around the back but a nice brushed-aluminiym box that sits towards the front of the "foot" giving you some front-facing ports.