I don't see the point in changing the pin lay out of the standard usb-c socket so people have to buy an extra OTG adapter to make it work with aftermarket cables.
My understanding is that the standard USB-C allows for bi-directional power sharing which was part of what OTG did for 2.0, but still does not incorporate the features of OTG that allow for switching between host and device modes and thus still requires the use of an OTG cable for those instances. It may be more a matter of it could have been wired without OTG in the case of the XD-05 as I am not sure in what circumstance it would be serving data back to another device, but maybe there is a use-case for it that made Xduoo consider it instead of just using a device only connect.