There are asteroid belts, and there are collections of bodies in resonant orbits, such as Trojans. Aren't these separate things?
The recent evidence for the 9th planet is an alignment in various parameters of known Kuiper belt objects. Conversely, few or no objects are known in the parts of the belt that would be swept away by the hypothetical planet.
If the Kuiper belt straddles the gravitational neighborhoods of Neptune and this other planet, and its populations reflect their influence, is it really a belt at all? A lopsided belt? Or just a region with a collection of distinct populations, each associated with a dominant body?