Definitely sounds like an inside job to me. The man slipping down to the wine cellar while the waiter left to get more food is just the cover story. It would take time for him to go through the wine cellar by himself and find the most valuable wines from among the others. Also, how could he, by himself, manage to bring all those bottles upstairs without being noticed? As someone else here pointed out, the guy would need at least four large suitcases to carry the bottles. Given the weight and the bulk, he couldn't do it in one trip. My guess is that the wine had already been removed before the couple sat down for dinner. The waiter and the couple were in on it.
It is a good theory.
The wine may have been sold to private collectors, for cash, piecemeal. Then, whoever did that needed to cover their tracks, so this story was concocted.