I thought that the backstory was interesting, especially because Reed kind of mixed religions/belief systems together. Let's see who we have:
1) Osiris, Set, and Isis (Thoth as well?)-- Egyptian gods
2) Dionysus -- Greek god
3) Moses -- Bible
There's probably more, but you probably get the idea. Plus there's also mention of other people in these chapters, like Thermuthis, daughter of Tutankhamen, who adopted Moses, Jesus, some more pharaohs, etc. I thought it was interesting that Reed used elements from both polytheistic and monotheistic religions to create this story. I have two ideas about why he would do so. I'm inclined to believe that doing so allows him to make a commentary on monotheistic religions, since it seems like that's what he's been doing throughout the novel, especially since he uses a lot of aspects from voodoo and loas. On the other hand, maybe he used all of these characters as a way to relate Jes Grew to everyone. The Book of Thoth and dancing isn't contained into just one religion, it's more universal, and as we've seen, it can impact everyone, so maybe this story adds to the background of Jes Grew.
I don't know, I feel like I'm getting this completely wrong, but that's just what I gathered, my first impression, whatever you want to call it. I definitely want to analyze these scenes more closely because I think there's a lot of interesting information that can reveal more about the book as a whole.