You’re not frikkin’ wrong. Planet Earth mines every last bit of it’s fissile material, wraps it up like some giant cooking cube full of spices, and sends it on a mission to the Sun, to save life and everything as we know it, by kickstarting a mini big bang.
So this giant 16km wide cube of fissile material is now at the surface of the sun, right? And somehow, inside this payload cube, the computers and electronics and air-conditioning are working just fine so that the 3 human beings left alive are still nice and comfy in their 72 degrees Fahrenheit enivronments.
And then at the end, the hero is standing on his gantry walkway as his mini big bang starts kicking into gear, and the ACTUAL SUN ITSELF starts burning through the walls, but our hero stays alive and puts his hands up to the flames itself? He’s inside the sun and looking at the pretty flames? Wearing jeans and a t-shirt? All this, after many earlier moments in the film instantly burnt stuff because they were now so close to the sun?
This is a film that should never have been made. It started out with promise, and went down hill the moment they diverted their mission. The ending was PISS WEAK. That’s why the OP asked what the ending was all about. Because it was PISSWEAK.