I wonder if the author can even be bothered to explain how the phoenix and dragon empress/emperor sources got all the way to the small closed world where Yang Kai was born. Yang Kai did get the wordless book also from that small closed world, so one might speculate that Myriad Demons Heaven dude, the so called Demon God, had something to do with all of it. Like he had, like sneak thief (truly Yang Kai's spiritual ancestor), snatched the weak sources after the inked giant had been sealed. Then Demon God (Mo Sheng, whatever was his name) had ended up fighting with Ruo Xi's ancestor, was defeated and his soul clones spread around eventually, taking the book, the Demon Eye of Annihilation and the two sources to pretty much the farthest reaches of Star Boundary, down to the lowest sublevel.