The hours you see posted are only for attractions and stores.
The park itself (in terms of you being able to stay) is MUCH later than the hours posted.
Last week we stayed at the MK nearly 2 hours after closing to take photographs. Not only were we welcomed to stay, the employees offered to take our photos for us. We were the ONLY ones in the parking lot when we left and the ONLY one on our monorail (I think we got the last monorail out).