>>1056A doomed timeline arises from something happening that wasn't what "needed to happen", so it's sort of vague.
However, Aranea's stated goal here was to deliberately create a doomed timeline, and then heal it, because she sees the doom as a sort of bad luck or irrelevance, and sylph of light powers would maybe fix that? It's not a very well thought-out plan. As we can see, now.