What decides how good a slingshot something is isn't its density or mass (provided the mass >> than that of the object being slingshot), but rather its velocity

What decides how good a slingshot something is isn't its density or mass (provided the mass >> than that of the object being slingshot), but rather its velocity relative to the sun. Planet 9 travels very slowly relative to the sun, and so is a poor slingshot, regardless of its density.