Gwern's typographical conventions are idiosyncratic, and you might not have noticed that he linked the original Scientific American article in his text. Reading the article now, I don't see how your interpretation is defensible.
It doesn't mention exploding the earth, and while there is a little ambiguity, as Gwern does imply, they are describing a recent paper that concludes that large fission reactions are simply impossible: "They found instead, that instead of building up to a grand climax, it runs down and stops like an unwound clock."
The final line of the caption is 'Readers made insomnious by "newspaper talk" of terrific atomic weapons held in reserve by dictators may now get sleep'. At least superficially, that sure sounds to be more about atomic weapons being impossible than about whether the chain reaction would consume the entire earth.
It doesn't mention exploding the earth, and while there is a little ambiguity, as Gwern does imply, they are describing a recent paper that concludes that large fission reactions are simply impossible: "They found instead, that instead of building up to a grand climax, it runs down and stops like an unwound clock."
The final line of the caption is 'Readers made insomnious by "newspaper talk" of terrific atomic weapons held in reserve by dictators may now get sleep'. At least superficially, that sure sounds to be more about atomic weapons being impossible than about whether the chain reaction would consume the entire earth.
I think you are confusing the actual linked article with Edward Teller's later argument that a nuclear fission explosion might ignite the atmosphere: https://www.realclearscience.com/blog/2019/09/12/the_fear_th....