I didn't state it wasn't. I stated Hagoromo did not write it, but instead Indra and the Uchiha. Fully read and comprehend peoples' statements before replying to them, please..
Hagoromo doesn't said in detail but it also doesn't state he didn't write.. To me, I'm sure Hagoromo did wrote it & not Indra or his reincarnation due to:-
1) Madara "ACTION" from beginning (steal Hashirama DNA) until he awakens third eyes
2) Hagoromo itself said "I left behind the Stone Tablet", this must for some reason..
For Indra or others Uchiha, I'm very sure he didn't wrote it, if not Indra or others reincarnation before Madara would do as Madara did right now...