Like the rest are saying, the issue is that this is before we really had a solid definition of what it meant to be a Sith. At that time, Palpatine was viewed more like an evil wizard because the idea of Sith was not really solidified. However, with the name Sidious now being Canon, I would imagine that he would think of himself as such. It would be interesting if someone rewrote the Dark Empire comics with what we know now.