being an astronomer myself(astrophysicist), i see this without any factual basis.
first, if the Nibiru is indeed a planet, then it would have been gravitationally locked into its own star system. the closest star system to our sun is Centauri star system(Proxima and Alpha) and that is about 4light years away(that is roughly 45trillion kilometers). these star system are more massive than the sun so they would have displayed more gravitational attraction. you know, same cliche when we were in college studying physics, F = G(mass1*mass2)/D squared, amount of gravity to be proportional to the mass of the object and the distance between it with another object. same Newtonian Dynamics applied.
second, a planet to be called a planet has to comply the planetary conditions. does it have an atmosphere, what heavy elements are there, what gaseous particles are present, etc. if this is smaller than our Moon, what difference does it have with our Pluto and Eris which are essentially not a planet.