It doesn't have to use HID for Nintendont to be able to have support for it. HID is a very common and very open standard, thus it made sense to put the time into supporting it in Nintendont. Xinput is a proprietary standard that is pretty much just for 360 controllers, so it's not really worth trying to add in support for it when the more widespread HID is already supported. This adaptor, however, even if it doesn't conform to the HID standard, would be more enticing device to support as it will be a fairly common peripheral among Wii U owners--especially those who wish to play gamecube games--that supports four original gamecube controllers.
Even if the Nintendont developers do not choose to support it, I bet you someone (maybe me if I can find the time in the future) will look into making an HID adaptor for it if, for nothing else, to get it to work for Dolphin.