He's correct. Remember how much further out planet 9 would be from the sun than Jupiter is. The speed at which an object orbits is related to how far away it is

He's correct. Remember how much further out planet 9 would be from the sun than Jupiter is. The speed at which an object orbits is related to how far away it is - note that the article said it has a really long year, too. Consider how far away geostationary satelites are compared to the ISS. The ISS goes around the planet several times a day, but geostationary orbits by definition complete one orbit in one day.