Only place I see that he's LD is when he flees, and the storm makes a wasted command + where, before following. He claims that he spoke to him while he healed up. So to me, the only sign is that when he didn't move out after the storm giant fled since he was convulsing. But at that point, the storm giant was badly hurt as well, a underhand strike would have finished him.

Nah, good log, well fought. But instead of you being lucky that he didn't manage to escape, you got unlucky and killed some LD dude.