Upvoted, as this is an excellent point. The IAU definition makes serious presumptions about our level of knowledge. Could the IAU definition be used at the time of Pluto's discovery? If not, what does that say about using this definition for future discoveries?

Note that I'm not saying that I think that dwarf planets should be categorized as planets (there are too many), but that the definition shouldn't depend upon us having complete knowledge of every object in a newly discovered potential planet's neighborhood.