Early prototype "mules" are typically just cobbled together donor car parts made before any tooling is available for the production body to test other parts first like powertrain, brakes, suspensions, electrical, cooling, etc. where having the production body early on isn't super critical. Things like the wheel base and tire sizes are probably correct but the Mach E body is probably just a place holder because it is the closest type of vehicle (i.e. an EV) to whatever is being tested.