On the other hand the book that claims Septim was from Alcaire also contains factual innacuracies and is referred to as the Heresy in the title.
Yeah, but the alternative of a mortal coming from long-dead Atmora is physically impossible. On the other hand, the Heresy's account of Wulfharth explains the Atmoran origin story.
Tiber Septim is essentially three people. The General Talos, The Battlemage Zurin, and Hijalti who actually became Tiber Septim.
General Talos' given name is Hjalti, except when Wulfharth was standing in for him. According to the Heresy, which is a lamentably incomplete bit of lore.