It also applies to parts, not just finished vehicles. That’s one of the most impactful parts of it. It’s specifically to stop the “car is 99% built when it is shipped in, and American workers just tighten a few bolts to call it finished” scenario that you described.

I think this may be a “broken clock is right twice a day” scenario, because it seems to actually be targeting the methods that auto manufacturers used to skirt taxes.