I cannot tell you why the dealership employee made the statement he did, but I can tell you the dealership does not issue 1099's on manufacturer's rebates. Primarily because the dealership would have no responsibility since the rebate is from the manufacturer and in fact is not taxable income to begin with. Depending on state sales tax laws, the consumer may or may not pay sales tax on the rebate but again this would have no connection to a 1099.
One must remember that some dealership employees can and do get totally confused about how some things work just as do some Forum readers. Until they get corrected on their misunderstanding, I have seen them pass along some very stupid advice. I sold tens of thousands of new vehicles with rebates in multiple dealerships in multiple states for more than forty years and can assure you I never once issued a 1099 for a rebate!