There is
one source of the kinako tuna story.
Supposedly, the story says that some generals went to a restaurant and requested a dish that's completely unheard of, hoping that it would give them inspiration to create a winning strategy against their enemies. The chef made this weird kinako tuna, and the general was pleased by this, and then repeatedly orders it as some kind of good luck meal. Other people heard about it, and then the food item spread.
The story is actually quoted from another blog, but the platform has been shut down so the original entry is not viewable. I can't find any archives or other platforms quoting the story.