It appears there may be a misunderstanding in the question. There is no particular significance to Atrocities
The Wikipedia quote is a bad summary of the article it references, which makes two separate points run together.
The article opens with the changes to the poem Atrocities, a choice made by the journalist for a number of reasons. The poem is fairly well known. It has shocking, evocative imagery. And the draft version makes clear that the changes are substantial, with the original being even more provocative. All of these things make it a great lede for a newspaper article.
The quote about the "treasure trove" in the article is much further down. Set in context it is clear that the person who said this, Sassoon's biographer Jean Wilson, is referring to the entire collection, not just a single poem.
Among the Sassoon material is a notebook with almost 50 previously
Sassoon's biographer Jean Moorcroft Wilson said: "This is very
exciting material. I want to rewrite my biography and I probably shall
be able to get some of it in. It's a treasure trove."
It's also worth pointing out that this can be read two ways. Although there's an implication that the discovery has made Wilson want to rewrite her biography, it's not concrete. "I want to rewrite" could be taken as reading that Wilson was planning a revision in any case, and is excited to also include the new material.
Indeed, it then goes on to make an entirely separate point about Atrocities, which makes clear that it's only part of the wider "trove".
Commenting on the Atrocities draft, she said: "The publisher,
Heinemann, wouldn't let him publish it.
The Wikipedia summary runs the "trove" quote directly after its description of Atrocities, making them seem related when they are not.