https://www.wired.com/story/china-conquers-mexican-automotive-market-and-the-us-is-worried/
State department hand wringing that China is going to use the US trade agreements as they were designed is 
I fully expect them to redo these with "except china" clauses.
yeah. From what I know about japanese imports even if you can get them to the US you can't register them if that model of car was never sold here (and maybe even if it was, idr), since technically it never passed various US safety tests that are mandatory. There have been lots of exotic cars crushed for trying. If it's older than 25 years though you get an exemption from the safety standards.
ready and waiting to buy a 2024 BYD car in.... 2049
But those Chinese EVs are sold here, hence the tariff being in place, right?
I don't think they are, and honestly I think it has to be a VIN that was actually manufactured for US market even then, but I'm not certain. Their USA site only lists one model and has no indication that you can buy it: https://www.byd.com/us/car/han-ev
Edit: some JDM forum guys claim you have to get it in writing from the manufacturer that the imported model is the same (or "substantially similar") as the one sold here, for the purposes of not needing to re-pass safety testing. Which, good luck with that unless you're Bill Gates (okay he actually ended up directly lobbying the government for a carve out for rare supercars, but still)
to the best of my knowledge the tariff was raised to make the investment in crashing a bunch of cars to satisfy NTSB standards less attractive. cost to consumers is doubled shrinking the potential market (even if they would still be competitive with basic econoboxes sitting at $38,999, the market shrinks considerably compared to a car that would fly like hotcakes at $14k but has to be sold at nearly $30k due to tariffs.)
The tariff is in place to prevent Chinese EVs from setting up shop in the first place. Taxed at 100%, no one will buy so why bother selling here?