Well, Sisko hadn't actually "just finished" being an XO. Immediately prior to being assigned to DS9, he was at Utopia Planitia, involved in the Defiant project. We know that his old CO, Leyton, felt Sisko needed a push to keep his career going, so recommended him for the CO position at DS9. It wasn't that important of a position originally, but Sisko probably needed a chance to be the CO of something if he was going to move forward.
What's the promotion requirements, though? We know Sisko was a lieutenant commander in 2366-2367, and that he was a commander in 2369, but we don't know if he was promoted immediately after Wolf 359, or if he was promoted to commander just prior to being posted to DS9. We do know he made captain in 2371, so he was a commander for between 3 and 5 years, which may be the minimum time-in-grade required for promotion to captain.