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Jang Jin-Sung appears to be the first author to describe a coercive transition from Kim Il-Sung to Kim Jong-Il. Rather than being common knowledge, his claim is a newly well-known story told by a single source.
Whether he is a credible source is harder to assess. 38north.org has not written about him. Mr. Jang was also responsible for some other sensational, unsubstantiated claims previously reported by VICE. Whatever his goals are, he leverages dramatic storytelling in Western media as a means of generating publicity and book sales.
The Washington Post wrote about his transition claim that "documentary proof is likely to be unavailable until the Kim dynasty collapses". The opaque nature of the North Korean state prevents independent verification, so his claim cannot yet be proved nor disproved: additional information would be news.