This seems like it. If you paid $29,000 for something with a $36,500 MSRP because of a $7500 credit and a few years later it's worth $22,000, the amount of depreciation you're calculating by starting from MSRP and the amount that the buyer actually experienced are off by more than a factor of two.
Meanwhile the credits caused more new sales than there would have been otherwise, which means more cars of that model available in the used market, and supply and demand is still a thing.
It's not evidence that people don't want them, it's evidence that if you subsidize something the price comes down.