TFA contains some bold claims given that the planet's existence has not been observed, but instead comes out of work they did to make their mathematical model to work:
"The researchers, Konstantin Batygin and Mike Brown, discovered the planet's existence through mathematical modeling and computer simulations but have not yet observed the object directly."
"Effectively by accident, Batygin and Brown noticed that if they ran their simulations with a massive planet in an anti-aligned orbit—an orbit in which the planet's closest approach to the sun, or perihelion, is 180 degrees across from the perihelion of all the other objects and known planets—the distant Kuiper Belt objects in the simulation assumed the (correct) alignment"
I'm not saying they are wrong -- I hope they are right! But, these are bold bold claims given the present state of the evidence. I mean, bugs in their model could also explain why an extra Neptune-sized planet is needed...