The cars only appear short due to compression caused by the lens. My son was taking the photos with the 1Dx + 100-400L II telephoto zoom using the "long" (400mm) end. The photos would have been better if he had been using the "short" (100mm) end.
The best camera + lens combo would have been the 7DII + 16-35L or 5DIII + 24-105L wide angle zooms. The 5DIII was unavailable as it was attached to the 500L telephoto + 2x III teleconverter for bird photos. I had forgotten the 7DII at home, but my old 7D was sitting in the pickup where it was doing no good.