As a local contractor that has done a great deal of work at WDW, and in particular the Magic Kingdom, overnight...I can assure you this is not the case.
The rides are not operating all night for the castle guests to ride at their leisure.
An hour after the park is cleared of guests, you would be shocked at the activity going on inside the MK. And it's not just the workers...there are tons of vehicles driving all over as well. Everyone in hard hats and reflective vests. Streets are lit up like it's daytime. I doubt they would let guests walk around, even if escorted. Too dangerous, beleive it or not.
I have pictures of my personal vehicle parked on Main Street and driving through Tomorrowland.
While the rides do not operate, the background music is still on, and let me tell you, without the hustle and bustle of the large crowds of people, that music is LOUD....and annoying. Especially Tomorrowland. It will drive you crazy.
An hour prior to park open, as they are clearing the contractors out, and the custodial people have worked all night cleaning every square inch of the place, I will tell you that the Magic Kingdom is a peaceful and beautiful place...especially as the sun starts to come up.