The point of US sanctioning BYD is moot.
Wan Chuan Fu built BYD into the world's most vertically integrated car company and allowed Berkshire Hathaway 10% sake for one reason: sanction proof. Just in case you don't know, BYD even builds their own IGBT devices.
Wang Chuan Fu went in a totally different direction from Ren Zhen Fe in that
(1) Ren built Huawei into a true global company. Huawei favored US companies over local ones for supply. All were good and dandy before US sanctions. Now Huawei have to turn the ship 90 degrees.
(2) Wang Chuan Fu built BYD into a vertically integrated industrial enterprise from day one.
(3) Huawei's global revenue was on par with domestic revenue before sanctions.
(4) BYD has barely started its international drive.
(5) It would be downright stupid to claim an EV is a national security threat.
That being said, let us imaging US sanctions BYD the same way as Huawei.
(1) Tesla cannot escape. Not even Xi Jinping can find any excuse not reciprocating on Tesla.
(2) Tesla market cap is 7 times that of BYD, with lots of pension funds invested.
(3) When TSLA crashes, the collateral damages would order-of-magnitude larger than any harm BYD gets.
(4) BYD is becoming the 2nd largest battery vendor and holds patents on blade battery technologies.
(5) Warren Buffet still holds around 8% of BYD.
(6) BYD is one of few foreign companies that run a successful union-base manufacturing company in US.
(7) Without BYD, US would have to buy electric buses from other foreign companies. From Yu Tong???
(8) When news breaks out in China, all US car companies would be boycotted by people. BYD becomes a darling of Chinese identity in China nowadays. Anyone who dares buy a US branded car after would be called out in the Chinese social media (人肉搜索).
(9) I could list another 800 bullet items but you are getting my point.
Whatever you think about the US politicians, the last thing they would do is to wrack S&P500 or NASDAQ, period.