It seems the dealer thinks you are gullible and will believe anything they tell you. The dealer is not replacing the windshield themselves, it will probably be done by a vendor like safelight.. Check the lower right or left corner of the windshield. The factory trademark is on the passenger side. All OEM windshields will have the GM logo and if you have the optional Bose speaker system, it will say acoustic and that's the only windshield that has to be special ordered from GM.
If it's from a vendor it will be in either of the same two locations and will say "LOF" or "PPG". Since this is a 2013, I would doubt it's the car itself, unless you had an accident of some sort in the meantime. Even if they were to find a small dimple of some sort the mastic would take care of that. Look at the windshield as it currently sits on the car. You will see there is a gap between the glass and the car body. Knowing where the leak is coming from, take a strong flashlight and search that area and see if you can see if there is anything that could be causing the leak. Best of luck!
If it's from a vendor it will be in either of the same two locations and will say "LOF" or "PPG". Since this is a 2013, I would doubt it's the car itself, unless you had an accident of some sort in the meantime. Even if they were to find a small dimple of some sort the mastic would take care of that. Look at the windshield as it currently sits on the car. You will see there is a gap between the glass and the car body. Knowing where the leak is coming from, take a strong flashlight and search that area and see if you can see if there is anything that could be causing the leak. Best of luck!