This moment made zero sense for Finn.
Why?
Because his character arc isn’t about self sacrifice or the lack thereof. Everything about him running away from fighting was resolved in TFA.
No, in TFA he’d never sacrifice himself for a cause. For Rey maybe, which is the reason why he went to Starkiller Base and the reason why he was going to defect, but not for a cause. Not for something bigger than himself. It was still selfish of him, because he was running away from a fight bigger than him for personal reasons. His arc in TLJ was that, while at the beginning of the movie he wasnt willing to do almost anything for the rebels, by the end he was willing to give his life if it meant that the rebellion could live longer.