My local dealer said the same thing. Laura is the No. 1 or 2 GMC dealer in the country though so they may have different incentives. They are probably using some combination of combined incentives, employee discounts, etc. and may have some special kickback year end for No. of vehicles sold from GM directly. Pure speculation on my part.
My dealer said if it was allowed they would drive to Laura and buy every Denali on the lot and resell them locally for 8-10% more which I could see easily given the low inventory, local pricing and demand.
I told them to work it out or I was making a road trip so after a little wait I was able to order one and they found an employee certificate for me which puts it just under Laura pricing without a $1,500 trade credit.
Where there is a will there is a way. They just have to get creative if they really want your business. The only negative on my deal is the painful wait for GMC to ship but that isn't the dealers fault. And when I spoke to Laura they seemed pretty disinterested in ordering but in all fairness I never got super serious with them to that level.