They probably just haven't done it yet since it's a self certification where the manufacturer themselves runs tests following NHTSA test procedures. Importers won't import (and probably can't sell in many jurisdictions) without that self-certification and it's likely just not worth it for BYD right now with the 100% tariff.

I wouldn't be surprised if they've already designed the core components of their cars (like the chassis) that they sell in Europe to meet American standards anyway. Stuff like the height and orientation of headlights can be modified more cheaply later when they want to enter the US market.