Let me quote an article quoting the scientists (emphasis mine):
Spanish lead scientist Professor Carlos de la Fuente Marcos, from the Complutense University of Madrid, quoted by the Spanish scientific news service, Sinc, said: “This excess of objects with unexpected orbital parameters makes us believe that some invisible forces are altering the distribution of the orbital elements of the Etno, and we consider that the most probable explanation is that other unknown planets exist beyond Neptune and Pluto.
“The exact number is uncertain, given that the data that we have is limited, but our calculations suggest that there are at least two planets, and probably more, within the confines of our solar system.”
. . .
One problem is that the theory goes against predictions of computer simulations of the formation of the solar system, which state there are no other planets moving in circular orbits beyond Neptune. But the recent discovery of a planet-forming disk of dust and gas more than 100 astronomical units (AU) from the star HL Tauri suggests planets can form long distances away from the centre of a solar system.
The answer is straightforward: There is little evidence for it, as "the data is limited".
Six months after this question was asked, Batygin & Brown (2016) (arXiv) showed indirect evidence for the existence of a ninth planet with a semi-major axis of ~700 AU and an eccentricity of ~0.6, swinging it as close as 200 AU to the Sun and as far as 1200 AU away from it.
As of several months after the paper's publication, no direct evidence of Planet 9 has been found (as expected), but the Subaru Telescope is expected to play a major role in the search for it. Hopefully, observational data can support the models in the years to come - within five years, perhaps, according to Brown.
The new results aren't quite the same as the results of de la Fuente Marcos & de la Fuente Marcos (2014), which are cited in the question (and in the paper by Batygin & Brown). For example, the original claim posits a planet with a semi-major axis at ~200 AU, in addition to another planet with a semi-major axis at ~250 AU, in a 3:2 resonance.
Furthermore, Batygin & Brown explicitly address and criticize the results of de la Fuente Marcos & de la Fuente Marcos and the similar studies by Trujillo & Sheppard (2014) (pdf here), who proposed the use of the Kozai mechanism as an orbiting body is perturbed by another body with a greater semi-major axis. In particular, they note that the orbits of the two planets would have to have orbital parameters within a small range of values. Also, another large body (e.g. a passing star) would likely have been required, which would have produced effects that conflict with observations.