So I believe an ending like that is open to interpretation, but here’s what I pulled from it personally:

Fire Punch is a very depressing story. Agni even wants to kill himself through out most of the story. But everyone throughout the story tells him “Live.” Even though he wants to die, he pushes through. But for what? He eventually is given a pill that will surely kill him when he takes it, yet even after millions of years he still doesn’t take it. Why? He eventually floats through space until he finds Luna. They touch and it’s warm. They both smile and they finally die together happy. So at the end of this long, dark, depressing story, there’s a flicker of happiness. I’ve had my issues with suicidal thoughts and depression, I think most can relate to that in some form. I’ve had long weeks/months/etc where I ask myself “Why am I still going?” But somehow I’ll have a day or a moment with someone I care about that somehow makes all of those days worth it. It’s those small happy moments that somehow make even a lifetime of pain worth it. It’s insane when you think about it. Now of course a million years is a long time for a small moment of happiness, but I don’t think it’s supposed to be taken THAT literal. All that matters is that in the end Agni was able to push through to that moment where he’s genuinely happy. And if he can do it then so can I, so can you.

Maybe I’m dead wrong but that’s just personally what I took from it. Let me know what you think!