Who knows, maybe they decided to start on DS9 early.
You know, the old "strike while the iron is hot".
I find that unlikely. Remember they have struggled with the immense workload of remastering TNG, so much so that they had to give the even numbered seasons to an out of house company. Doesn't make sense they would start work on DS9 in a situation like that. At least not unless they massively increase the amount of people they employ.