Who's the First Father? The inventor of sorcery?
Perhaps a creator/progenitor myth?
That's what I assumed when I read that. I've always thought it was very suspicious that there are virtually no creation myths whatsoever, especially within the Inrithi faith or even the Fanim.
I think that is a reflection of Bakker's worldbuilding process, and specifically the fact that he did not seriously flesh out Inrithism and it's theology (to whatever extent he has done that at all) until well into his writing of TWP. If you recall Inrau's unfortunate meeting with Not!Sarcellus and the Rape Pigeon, there was a brief exchange in which Inrau asks Auranx why he does what he does, and Auranx laughs and replies "You worship suffering." This enrages Inrau because "it didn't understand!", but it is never expanded upon later and we still know next to nothing about Inrithism. I think that Bakker originally intended Inri Sejenus to be a much more exact Christ-analog than he has turned out to be, complete with some sort of atoning vicarious death, hence the "suffering" Auranx mentioned, but that Bakker dropped that idea when drafting TWP in order to improve Kellhus' plot arc with the Circumfixion (you'll note that there is no comparable torment in the life of Sejenus that we know of, although he does get a trial before evil King Shikol, presumably an expy of Pilate). If I am right, this would also explain why Auranx's comment angers Inrau so: the Inchoroi's artificially deadened emotions prevent him from recognizing the difference between bearing suffering to save others and his own psychopathic sadism.
If all of that is on target, then it is possible that Earwa's version of the Biblical Adam is not discussed in the books because Bakker never worked on that aspect of things, just as he hardly worked on Inrithism's central figure beyond the necessary points needed to serve the narrative purposes of TSA.