That's wrong. He literally told Sylvanas that he wanted her to he a co-worker for him, and that he wouldn't do any of that "serve me" shit on her. It would be the two of them remaking the cosmos as equal partners. That's what he told Sylvanas, and that's what Sylvanas thought throughout the first half of SL.
Zovaal was gonna enslave everyone else, but to Sylvanas, it was for a "greater good".
However, in reality, Zovaal duped her and simply saw her as another tool for his plans. There was no "equal partnership" involved at all. The Devil quite literally tricked her. Not to mention he fooled her motive-wise as well, saying shit like "oh yeah, we'll reforge the cosmos to be just and fair" even tho he actually wanted everything to be chained up, full of torment, etc.