I’m not sure where this conversation is going.
There were three films made. Out of the infinite number of narrative possibilities, a small finite number were written in stone, while another small finite number were left ambiguous and open to interpretation, and the remaining infinite number were undisturbed, from which future ideas for future stories could be gleaned.
Logically, the Emperor could have turned out to be a sith, or he could have turned out to be a figurehead. But if he were to be a sith–which indeed was the case–Tarkin couldn’t have known about it, because of what he said in the original film.