he says people should accept that "we have eight planets and only an object bigger than Mars could be considered to be a planet in the future".
Now, I could see a case for "there's tons of minor planets out there, and we made a mistake in including Pluto because we thought it was much larger. To be a planet any body will have to be at least as big as the smallest of the classical planets, Mercury." It's still pretty arbitrary but it has a certain historical resonance.
I could see a case for arbitrarily declaring anything beyond the gas giants as always just a KBO or Oort no matter their size.
But 'a planet is anything as large as Mars... except, um, Mercury' just seems completely illogical.