I see. However, they definitely sold those packages way in advance and someone must have made a decision at some point at time that they would at the very least have soft openings going on for those guests if they weren't ready to officially open yet, which is exactly what happened. I was one of the people who purchased a WWOHP vacation package, and I went from being shocked and confused when they announced a grand opening date after my vacation dates to relieved and surprisingly pleased when I discovered that although WWOHP wouldn't be open all day, it would be open in the mornings to vacation package guests only. A few hours in a minimally crowded DA beats opening-day DA crowds hands down.
I would think they could fill their on-site hotels even before the grand opening if they began selling WWOHP-Diagon Alley packages that guaranteed guests would be able to visit DA at least a few hours each morning whether it was officially open or not. The only reason I can imagine they wouldn't start selling DA vacation packages is if there is some important aspect of DA (e.g. Gringotts, Hogwarts Express, or the new Ollivanders) that is so completely dysfunctional that they are completely unable at this point to predict even when it will be ready for partial-day soft openings.