Is it credible? Sure. If you use it as what it is, it is credible: it is a fictionalized account of one person's experiences of the horrors of war. If you treat it that way—as fiction (not fact), and as a retelling of what one person saw-- yes, it is credible.
Now, if you're asking if this text should be taught as a history of the war, or if it gives a complete account of Japanese-Korean relations, the answers are no, and no-- but then it does not claim to do so. Therefore, using it by itself as a textbook for studying this period would not be appropriate. Instead, you might try placing it in context with other accounts of the war, and discussing how historians determine truth, accuracy, and what weight to give differing accounts.