From anything I've heard, Disney does not usually close the park early for press previews of Halloween Time. They have media days for really big, new things (which Halloween Time is not). such as Cars Land, the Diamond Anniversary, or Frozen Fun, etc., and those might require a special night set aside. I think that the smaller media days and press previews are not as disruptive (or at least I believed so)?
The supposed reason for the early closing on 9/10 is a cast member event (Legacy Award party or Service Celebration party) that happens every year.