Is the fact that the simulated subject is a human important for the proposed thought experiment, besides that it activates all sorts of wrong intuitions about free will and makes the lookup table unimaginably huge or even infinite?

Is it clear that an accurate X exists?

It is not, why should it be? By assumption the subject does whatever the GLUT predicts but it doesn't follow that the GLUT includes a proposition "if the subject is confronted with the information that the GLUT predicts that he will do X, he will do X".