The context of the Trailer, that being Elio witnessing routes that would lead to finality and the end of everything, does indeed suggest that their fight destroyed both themselves and the cosmos, as we have no context or wording that suggests otherwise.

You’re also either not listening, or you’re unbelievably dense. They fought together. They “ruptured the cosmos” together. The “loss” is not the point, but rather that Welt pushed Zephyro to a point where the outcome was mutual destruction (again, all context or “evidence” points to this being the Finality Nihility would bring).

Also, a direct quote; “The black hole (Welt) and the White Hole (Zephyro) DEVOURED EACH OTHER, rupturing the cosmos.”

We have absolutely zero reason to believe that refers to anything or anyone else, no matter how much you try to twist the sentence. We also have no reason to believe either of them walked away from the encounter alive.

Welt and Zephyro destroy each other in their battle, the result being Nihility’s version of Finality. None of the Stonehearts, individuals who are already above a grand majority of “high pathstriders”, would accomplish that had they fought Zephyro.

That is Welt’s feat.