An asteroid with rings and likely shepherd moon is only 250km across and the new dwarf planet 450km across is described as:
Trujllio and colleagues estimate that the new dwarf planet is relatively small -- about 450 kilometers (280 miles) in diameter, which less than the driving distance from Philadelphia to Boston. It's probably ball-shaped, he said.
So why is this not a major planet such as Mercury, Venus, Earth and Mars? Trujillo explains that a bona fide planet is big enough that other objects in its orbit will be sucked into it gravitationally. A dwarf planet is not big enough to become gravitationally dominant; it's too small to pull in objects in the area of its path.
There's a discrepancy here.
Also, the similar inclinations of the two distant objects could be due to the exotic dark matter hypothesis rather than a 10x super-earth planet:
In the most speculative part of today's announcement the astronomers noted that 2012 VP113 shares a similar inclination in its orbit to the previously discovered Sedna,which Trujillo also had a hand in discovering, and which was originally termed 2003 VB12. They suggest that a much larger planet's gravity could be corralling both objects.
They suggest that this as yet unseen gravitational sheep dog would be huge: perhaps 10 times the mass of the Earth, and orbiting a few hundred times further from the sun than our planet. Such 'Super Earths' are now routinely found around other stars. To find one in our solar system would constitute as much of a shake up as the discovery of Uranus, Neptune or Pluto did back in 1781, 1846, 1930 respectively.