Comment by talldayo

It's not easy. Lithium-ion batteries are designed to withstand heat without presenting an immediate or non-obvious threat to the user. The easiest way to cause a pyrotechnic discharge is to penetrate the battery itself, and even that isn't terribly explosive (here's a laptop battery "exploding": https://youtu.be/oieH2wwDGzo )

If someone did try heating up your phone to implement such an attack, you would feel it burning through your denim pockets long before it hits 210f. Futhermore, both phone SOCs and battery firmwares tend to implement emergency shutoff contingencies for when the phone overheats. Without prior tampering, nothing will really behave like it does in this attack. It is 100% a supply-chain threat.