I don't really want to get too much into the technology debate, suffice it to say that the Star Forge is significantly more advanced than anything seen since, and it was built some twenty millenia before the conception of the republic itself. Technology doesn't necessarily progress in a linear fashion. Besides, the Coruscant in episodes 1/2 was probably more impressive than anything in KotOR.
However, the Sith as a species didn't exist even at the time of KotOR. There's a fair bit of information on the history of the Sith written
here.