They don't explain it in detail, but the reason it's consistent with Planet IX [1] is its orbit has specific properties that require a near pass with a large body in the outer solar system (basically the definition of Planet XI). That seems like it may be easy to do by other means, but if you've played Kerbal Space Program you know how difficult it can be to change all six orbital elements in the way you want to. For example, a close pass to Jupiter could launch something extremely far our, but it would not change it's perihelion or closest approach to the Sun. Like a Homann transfer, going to a larger orbit requires at least two "kicks": one to kick it into an ellipse and a second at the outer point. Planet XI would give the kick at a far distance to get many of these curious Trans-Neptunian Objects out to crazy perihelion distances like 65 au. There's also a specific effect to the inclination and argument of perihelion, but I don't understand how these work as well.

The amount of sky we have to look for Planet IX is enormous (and given how faint it is likely to be), and the evidence so consists of about ten of these TNOs with these orbits (not many, but it's very hard to make these orbits). But there are potentially hundreds of bodies with orbits like these waiting to be found by a the soon to be constructed LSST, which is an 8 meter telescope that will survey its entire sky every 3 days. With many more of these orbits to predict from, we may really narrow down the search space to look for it, and ultimately test its existence.

[1] Predicted by Konstantin Batygin and Mike Brown, the latter of which played a big part in helping to disqualify Pluto as a planet by discovering Eris. I know from a lunch-talk that he greatly relishs the thought of both demoting the former Planet XI and predicting the new one.