Unfortunately, this is true. It's relatively new, as it wasn't happening when we were there at the end of December. On our last trip during the last week of May, they WERE holding cars until 8:45 (or 15 minutes before opening -- even on early open days, when only resort guests can get in anyway). It really irked us, since we always take a car and always drive. On one day we were the first car in the lot and still were well behind the crowd; barely made it in and to the tree before the opening ceremony ended. They've also roped off or stationed CMs at the stores on the left where we always cut through their walkways on the way to Africa.
Several things about this get to me:
1. Why do this at ALL on early entry day? They have to check your room key at the gate and if you're off-site you won't get in until the later time anyway.
2. If they are doing this to try give an advantage to on-site guests, they should have a lane for on-site guests only and simply check the dashboard parking signs you are given at check-in.
3. If they are doing this to try and help on-site guests, they need to make sure they have adequate bus transportation from ALL on-site resorts! We stopped using Disney transportation most times because it's too unreliable, especially trying to get to a park for early open.
4. Why block the smaller paths around the main one in the park? It just makes things more crowded (ie dangerous) and makes the guests who know what they are doing mad. Why should they care if a few dozen of us who know where we're going cut over to the smaller paths in front of the stores -- that aren't even open yet, so no danger of us running into anyone coming or going?
As you can tell, the whole situation left a bad taste in my mouth. It's as if they are TRYING to punish their most savvy guests.