Who sez up to 2/3rds isn’t Earth-sized, you? Big deal.
The evidence for an undiscovered outer solar system body consists of comet focusing (Matese et al); the large (Pluto-sized) bodies on very long, eccentric orbits (Brown et al); and the people in this article based their figures on a computer simulation based on retrocalculating the orbits of known bodies; others.
The perturbation of Neptune, detected not many years after discovery and the calculation of its ephemeris, has been attributed by some to a few bad observations, but that’s a minority view. My own minority view is, the perturbation was brief because the planetary body is either A) orbiting more or less in the ecliptic, but in retrograde, so everyone looking for it winds up looking in the wrong direction, or B) orbiting out of the ecliptic, and perhaps also in retrograde.
I don't care what Neptune in the privacy of its own home and why are these pervs spying anyway?
I totally flubbed that joke
Another consequence of a retrograde outer solar system body would be that it mitigates in favor of all other unknown solar system bodies having retrograde orbits, or having really elongated, kinda screwed up orbits (which has been observed in a few cases; plus all the comets).