I'll concede a point on the "heard from a vendor". Unless one of them would care to post the particular vendor contract details there really isn't more we can debate on that point.
So lets look at what is happening. Since Hot Box has become the official pizza of GenCon no other food trucks have been selling pizza. That means either the food trucks that sell pizza (I've found 4 on a quick lookup) don't feel they compete with Hot Box on quality/price/profit or are not allowed to compete at the venue. The former is an unlikely as a business that is failing in those areas is soon gone, and what pizza place doesn't want to go head to head and show they are the best.
The latter, a non-compete clause with a sponsor makes more sense. Hot Box would have invested a large amount of money in GenCon advertising and product placement. Part of the product placement would be to exclude other vendors from selling the same food.
Now to your last point that the truck vendors take as fact because they don't want to get in trouble, but the only reason that they would worry about getting in trouble is if there is a written agreement that says you are not allowed to sell 'x' food which points back to my latter point.
While the evidence is circumstantial, I believe the argument is sound. And as I said, unless someone posts the actual terms of the contract, we are all just spitting into the wind