I suspect that would be very much on the high side of what you could negotiate. Certainly was a couple years ago. You can still probably get something without breaking too much of a sweat if you'll close the deal right now.
From the article, it sounds like a 10% _worse_ deal is something dealerships manage to "negotiate" unsophisticated buyers into fairly regularly? It specifically mentions convincing someone into a monthly payment, then upselling them into worthless extras and worse finance that results into them paying 72 months at that price instead of 60 months as initially agreed. Just learning how to negotiate your way into paying the advertised price with market rate finance could seemingly make you 20% better off than someone who doesn't know how to do that.
