What you're saying makes me think this dealer is likely shafting people who actually placed an order for a car at MSRP ...by trying to get someone else to pay more and essentially selling it to the highest bidder.
Once they have an allocation, they place an order. If they are jumping your order for people who are willing to pay higher then MSRP, then there is a chance your order will never get place. Or once they do place your order, they market your car to someone else who's willing to pay over MSRP. At the end of the day, you don't start the buying process until the car is built and arrives. So they can be telling you when you order it it'll cost MSRP...but then the day comes to sign and they tell you there's a market adjustment....take it or leave it.
But just a heads up, if GM does not accept your order by the end of this month, you're likely going to have to wait for a 2023. The end of this month is the cutoff for GM to accept your order, if not it gets moved to a 2023. Those won't hit dealer lots until early Fall. If you need a car somewhat immediately, you may have to accept the markup. It's your wallet to determine what you think is excessive or not.