Disclaimer: I took a class under Bruce Cummings.

I do not know enough about North Korea to argue as to the nature of Juche itself, but Myers essentially constructs a narrative of the Japanese colonial period to suit his argument that Juche was derived from Japanese propaganda. He argues that from the very beginning the "educated elite" were co-opted by the Japanese and assimilated to the idea that "Korea and Japan were one and the same." He even uses a postcard of a three-legged race to back up his argument... while blissfully ignoring the implications as to why the figure representing Japan is a step ahead of his Korean partner.

The reality of the Japanese colonial period is that, like other Japanese colonies, colonial administration was inconsistent and changed depending on the period. Myers argues the educated elite "as they always are" were co-opted into collaborating with the regime while the "illiterate masses" were out of reach of the propaganda when in fact the Japanese went out of their way to disenfranchise the Joseon nobility in favor of educating the "masses" to create a means of feeding pro-Japanese propaganda to the populace and bypassing the existing power structure. The Japanese damn well wanted to have the Koreans accept Japanese rule peacefully, and they also wanted them to know that Japan was going to lead its unruly "younger brother" into the world, whether the younger brother wanted it or not. Myers uses anecdotal evidence to make it seem like the Japanese actively tried to organize the Koreans to be good Japanese, but the Japanese actively discouraged it, including forbidding the Koreans from adopting Japanese names and explicitly teaching the Korean populace the Hanguel alphabet (at least until the rise of the militarists in the 1930s.) In an effort to show the influence of race from the Japanese to the Koreans, he ignores the very real racism shown by the Japanese to the Koreans as well.

While certainly the assimilation policies started to kick in around the 1930s until the end of the war, Kim Il-Sung had already left Korea for China at that point. I would argue that Myers intentionally is making it seem like this Japanese assimilation and racial policy started from the beginning of colonization in order to make it seem like this was the environment that Kim Il-Sung was raised in, but in doing so it ignores the complex dynamics and internal politics of Japanese colonial rule, not to mention he is associating Japanese "fascism" with periods of time that Japanese historians refer to as the "Taisho democracy!"

While certainly Myers' book makes a lot of arguments, I suspect his desire to prove a point (and make a political statement, if his snide remarks against left-wing intellectuals throughout the book are a sign) at the very least contaminates his analysis of Imperial Japan as a vector for Juche thought. I would like a Korea specialist to chime in here on the parts of the book that I cannot comment on (which is to say, most of it).

Sources:
Kimura, Mitsuhiko: "Standards of Living in Colonial Korea: Did the Masses Become Worse Off or Better Off Under Japanese Rule?"