Because it's been tested to be way safer than other batteries.
https://evpowerhouse.com.au/blog/byd-blade-battery/

One of the primary concerns with EVs is battery safety, especially during accidents. Traditional lithium-ion batteries have been prone to overheating and combustion. However, the Blade Battery boasts several safety features, starting with its use of lithium iron phosphate (LFP) for the cathode material. LFP chemistry offers superior stability, even at temperatures as high as 930 °F (500 °C), making it significantly safer than conventional lithium-ion batteries.

That means it's generally safer in theory because it more chemically stable.

The Blade Battery has also passed industry-standard tests, including the “Everest” test, where a nail puncture is used to check the battery’s resistance to ignition during an internal short circuit. Unlike other batteries that charred or exploded during this test, the BYD Blade Battery remained cool at 30-60 °C (85-140 °F), demonstrating its exceptional heat management.

Practical test results confirm this, and damage that will make other batteries burn explosively, merely makes this battery warm.
But it's not 100% safe, there have been accidents that shouldn't have happened, but did anyway. This is being investigated, and it is still considered significantly safer than competing batteries.