I think it’s interesting when people say that Moon is an unreliable narrator with the implicit idea that Pebbles isn’t. I agree, Moon is an unreliable narrator. But that doesn’t make Pebbles de-facto reliable.
Almost everything we know about “karma” and “ascension” that’s not empirically seen is explicitly filtered through those two. And the rest is through other iterators. I think everything they say should be taken with the idea of their bias in mind, especially with the knowledge that almost all of them are bitter and/or uncaring about the Great Problem anymore.
We don’t know what happens when beings ascend. But Pebbles was a part of the Sliverist movement - which WAS explicitly “what if the way for Iterators to ascend was death - a non-traditional method that had no backing except the mysterious disappearance of Sliver of Straw.” It’s an ideology born of desperation - of the want and need to escape, but having the very nature of yourself be antithetical to the concept.
I don’t think that ascending is death, and I don’t love when the two are conflated. It’s kept purposefully vague what happens after the end-game cutscenes, and there’s a lot there to speculate on. I do think it’s - explicitly - not death as we know it here, But Pebbles was a part of a movement that conflated the two.