I have not read that article so it wasn't clear to me if they were talking about downforce from the spoiler OR about total lift at the rear axle. Thanks for clearing this up!

However, please observe that unlike with a wing, it is not true that when you add a spoiler the total lift is a sum of (basically unchanged) lift from the body plus downforce from the wing. Just as you said, the spoiler changes the airflow at the rear of the body. So the total lift in this case is NOT a sum of the "naked body" lift and the "downforce" from the spoiler. As a result, the decklid is not necessarily pushed down with a force of 100-150 kg as it is on a winged Cup car. Arnie may still have a chance