While this is a fairly informative site, you do realize it's not an 'official' site, right?
This site actually says that the resort to parks buses start running an hour prior to park opening, and that's completely wrong...they start by 6:30am, every day!!
So, even if the buses do start running to DTD by 8:30, they are not going to be as frequent as regular buses. And....the buses may actually be servicing Typhoon Lagoon, with a stop at DTD. There isn't much reason to be at DTD that early, but people are heading to TL for the day.
If I had to be at DTD for 9am, I would take a cab.