That isn't necessarily true, depends on if the rebates are stackable or not.
For example, I just got 0% for 72mo ( way longer than I would usually consider financing but hey 0% ia 0% ) with $0 down, dealer invoice pricing +1.5%, less $2,650 GM credit, $500 Costco gift card and i negotiated a thrown in tonneau cover as well.
If i didn't take the financing i could have had $3,600 taken off the truck instead of that incentive, but by my math for each 1% over 0 the i would have otherwise financed the truck the 0% interest promo is worth a little over $4,200 so I will take the finance and let my money earn a return elsewhere instead.
All I am trying to say is you need to take each deal at face value rather than generalize.