Martian manhunter and fire on Supergirl

So I noticed during the crossover finale, Martian manhunter makes an appearance where he shields a group of people from an explosion which includes fire. But he is vulnerable to fire. that's his kryptonite. Or has this not been addressed on the show? As it is, he's quite depowered on the show, like if the writers really utilized him, he'd be more powerful than Kara.