The movie didn't specify that it was a KBO; in fact, in contradicted that idea (at least in reference to what people are usually talking about when they say "KBO") by showing a craggy, spikey shape instead of a sphere. The implication of what they were saying was, to me, that it was from outside the solar system, or at least as far out as the Oort Cloud if not farther.

A KBO or Oort Cloud comet wouldn't just drift into the inner solar system, but it could be perturbed into coming here by another passing object. But if it crossed through the asteroid belt and perturbed some asteroids and small debris there, then it and that smaller stuff wouldn't both go toward the Earth; the objects that were already in the belt would have had momentum in a completely different direction from the big intruder's direction of travel, and their new vector would be a combination of that momentum and the big intruder's influence, not just the same as the big intruder's vector alone.

BTW, to whoever asked why use a vague or hard-to-interpret analogy instead of just specifying its mass: in the movie, the scene that the phrase "the size of Texas" comes from has two guys from NASA on the phone with the President, and one of them starts doing what you suggest, then the other says "It's the size of Texas, sir." So the purpose of the phrase isn't to quantify it with any precision but to convey a quickly-grasped impression to a layman.