That was the point of my post. One is specific to a particular show the other isn't.
It will be the specific fictional product taken from the specific fictional show, and how that fictional product is marketed, that will make a difference in whether there is a rights holder to concern oneself with or whether the product can be developed without legal risk.
Ideas are not protectable (absent a nondisclosure/confidentiality agreement), so using a TV show's fictional
idea for a product (flying cars) will generally not be an issue. Copying the product itself can be as can be using the show to promote the product.
Toyota has a patent on a flying car, too, so patent rights could also be an (unlikely) issue.