Well it say Revan have completely fall into the dark side
I believe that it says: "But no one knew how fall Revan had already fallen" as it was leaving the Mandalorian Wars. That, to me, hints at the fact that what we already know from Kreia & the Kotor comic is true: Revan found Malachor V and its Trayus Academy before the end of the war and him meeting with the Sith Emperor sealed the deal.
It also makes it very clear that Revan wanted to be his own man and had his own ambitions. He is already unique in the fact that he wanted to combine both the teachings of the Jedi and Sith to create something that just worked. His master-mind plan of getting the Republic ready for the "True Sith" still works-- he knew what to expect and he needed things ready.