I feel like this video underplays the significance of that flag for not just Korea but almost all of South East Asia, by a fair amount.
It is by no means an understatement to liken them to the nazi after learning about everything they did. heck some of the stuff they did in China was so horrifying that nazi officials were appalled. not really sure why this video focused to heavily on just Korea, when this would be something that would make a fairly large number of countries angry.
at that time japan saw itself as the rightful rulers of all of South East Asia, and maybe more, and they saw everyone that was them, as beneath them, less than human, and so they treated them as such. that flag was a representation of that idea, the central dot was japan, whole the lines coming off of it was their control over everyone else, which is one of the reasons they were forced to change it after losing the war. a fair amount of South East Asia still very much remembers what was done to them, and what that symbol represents.