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.
I would actually hope for it to plug into Thunderbolt to reduce bottlenecking. It's better to have all the devices sharing Thunderbolt bandwidth than the significantly smaller USB3 bandwidth.