The Kuiper belt is way too big to really be considered a "neighborhood". We're talking about an unfathomably huge area full of many orders of magnitude more objects than we have so far been able to identify.
A planet-like body could easily be clearing its orbit out there without us being able to definitely detect the absence of material in that orbital area.
Moreover, various "Planet Nine" theories I have seen estimate this theoretical object's size as 1.5-5x the size of the earth. If an object that big isn't qualifying as a planet by definition, it's the definition that's wrong.
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