They didn’t mention the theory that the planet isn’t of our Solar System and it was captured from another solar system.
My guess is, any planetary bodies discovered beyond Pluto will be moving in retrograde (or at the very least, at a substantial angle to the ecliptic), which is diagnostic of capture. This is analogous to the many small moons of Jupiter, and for that matter, Neptune's moon Triton.
A new theory is also making the rounds...
A micro-black hole.
SciTechDaily had a story just a few weeks ago:
https://scitechdaily.com/bold-plan-to-determine-if-planet-nine-is-a-primordial-black-hole/
A Harvard astronomer suggested we could easily locate the tiny flares that a micro-black hole would generate in the Oort Cloud.
Stephen Hawking theorized the existence of planet-mass black holes 50 years ago.