Ok, so go to the Pinephone battery spec (everything is publicly available) and point me to where it says the in-battery protection circuit protects the under/overtemperature condition.
In fact it's just the common protection circuit that just cares about over/under voltage and over-current conditions (mostly just a short, because the cuttoff current limit is 6-8A and battery's max recommended sustained current is 1.4A). So you can overload the battery 3-4x for however long you want, and that circuit will do nothing.
I wouldn't trust something as overly-complex as Linux to prevent fires anyways.