Human designed devices _also_ function in this regard throughout their assembly. Varying the definition of "function" for only one side would make the argument spurious.
Now, when we say that they have to remain functional at all stages of development, we mean like unlike your car, during normal development you can't cut various control lines and reroute them.
You can remove the brakes in autocad temporarily to let you tweak something else a bit easier. Evolution cannot do the same with RLN. Removing the RLN at any stage of development constitutes a negative adaptation.
Also development of car is external process to car itself. Most of developments of car goes on on models. ( which are not physically the same thing )