No, that's not it. They know exactly where the parts are. The dealership may lie and say they can't, but when an honest dealership gives you a list of every dealership with that part number in stock in a 500 mile radius it becomes pretty easy to challenge that lie.
The issue is dealers are independent businesses who own the parts they have in stock and can charge other dealers what they like. The warranty will only pay the cost of the part from the supplier, so if your dealership buys the part from another dealer at anything higher than that price they eat the cost.
So, they order the backordered part. Then (and this is important) *THEY CAN'T CANCEL THE ORDER*. So, if you find them a part, they don't want to use it because now they have a second part they allegedly don't need still coming they have to pay for.
Now, sometimes parts actually are out of stock everywhere. Etorque is a good example, and then it's just the mothership taking the entire supply and not releasing any for warranty or after warranty repairs.