BYD is indeed producing cars by the millions now. Any quality issues would be very apparent at this scale.

Also worth noting that they are very advanced in applying robotics and automation in their factories. Anyone assuming that this is all underpaid Chinese workers doing things manually would be mistaken. And that also means they are very good at maintaining the same level of quality through their production process. So, this is not surprising.

Also in general, the Chinese car market is insanely competitive right now. There's just very little room for manufacturers that deliver low quality products with lots of warranty issues.