Well, all they have to do is input your VIN to the key code website at GM.
This will give them your correct cut key code, cut the key and then use the system.
However, to program your key fob, they do NOT need a key, they can perform that process using the TECH 2 and invalidate all fobs and then reprogram them.
Document ID#1970253 is the GM procedure. Your dealership can access this document