No, it’s an argument he stole from white men the 1860s who found Ceres, called it a new planet, but then subsequently found Vesta, Pallas, and the rest of the asteroid belt and then had to come up with a new designation for them because no one wanted fifty thousand planets.

If we find a Mars-sized object outside the Kuiper Cliff (or actually find Planet Nine finally), that will be a planet. Still, the IAU’s definition is bullshit.