the Chrysanthemum Throne — from the sun goddess to the present, one line that never broke
Gold here is no metaphor reaching: the throne's emblem is a sixteen-petal chrysanthemum in gold, and its regalia — mirror, sword, jewel — are literally handed from one reign to the next as proof of succession. That hand-to-hand passage is the thing that "flows."
But the river's headwaters are myth, and the pamphlet says so. Amaterasu and Jimmu are legend; the eight reigns after him are the "eight undocumented" — names and dates with no recorded deeds, which historians treat as later invention. The line becomes firmly historical only around the sixth century, near Suiko. From there the count runs continuous — through centuries when shoguns held all real power and emperors reigned without ruling — down to Naruhito, the 126th.
So "unbroken" is the throne's own traditional claim, and it's true in the way that matters here: the line of succession was never cut, even when its power was. The gold kept flowing underground.