The Caterham press release reads nothing like the AutoWeek article. AW's Graham Kozak has changed the story significantly with just a few added words. That's just plain irresponsible journalism on the part of Mr. Kozak, as well as AutoWeek generally. Boo, hiss.
I see the federal regulations as fairly straightforward - until an engine or gearbox is installed, it's all just "car parts", in some stage of assembly. Once an engine or gearbox has been installed, it is forevermore a "car", even if the engine and/or gearbox is subsequently removed. They have to draw the line somewhere, so even if they move that line to include/exclude other equipment, it just changes the maximum level of assembly which "car parts" can be readily imported without crossing the line to be defined as a "car", and subject to significant restrictions.
NOTE: I am not an attorney, and have no relevant training or experience in any legal field.