I don't believe that is true. My understanding, in fact, is that the Rally1 car can even be scaled up or down from the road car. I think they _can_ use production body panels, but they're not required to. I believe the Puma is scaled down for Rally1 and the GR Yaris is scaled up for Rally1. I am having trouble finding a good reference for this right now, but I'll include a screenshot below.
WRC, the rule set that the GR Yaris was originally aimed at, required retaining the frame, roof, doors, hood, and hatch (you'll notice this includes the aluminum and carbon bits) while the suspension, drivetrain, and fenders could be changed.
This screenshot is from a search I did-- when I click on the link I don't get the actual article and I can't check web.archive.org on this machine.
I know there are some changes in the works to the Rally1 regs, but I'm not aware that this has changed yet, or that it is likely to.
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