Planet 9 "was first postulated in 2016 by Caltech's Konstantin Batygin and Michael Brown"?? Not even close. A long list of people over the past 80 years have postulated about the existence of a large planet far beyond Neptune. Many citing the exact same evidence listed in the 2016 paper. They were essentially called conspiracy theorists and their theories labeled as "debunked". The 2016 paper was simply the established science community's trying to claim credit for something they long ridiculed by citing the exact same evidence that's existed for decades. An attempt to say "Oh, ya , we knew that... in fact we actually discovered it" instead of simply acknowledging that they were finally willing to admit the validity of something they'd "debunked' for decades. The declassification of Pluto as a planet have them the cover to act as if their claims were new and novel by calling it Planet 9 instead of the 10th Planet, Planet 10, Planet X or whatever. They've since attempted to rewrite history by trying to convince everyone that Planet X was simple a baseless conspiracy theory about a sinister planet that spelled doom for Earth. It wasn't, it was a generic term used as a placeholder for an undiscovered but a highly evidenced large planetary body in the outer solar system with an off axis orbit around the sun.
I hate this aspect of science.