Either Tar-Meena is a rubbish historian,or she implies the -commentaries- are contemopary with ol' Tiber,and he wasn't.This either means that,before Haymon Camoran died,he wrote a book under the name Mankar and decided to name his child this,or he has powers over time.
Or someone else (possibly Haymon) wrote the Commentaries, and Mankar just slapped his name on them and claimed he wrote them all that way back, or he wrote them later in the Third Era and managed to make all the records show them being written earlier. Add in the family's secrecy during the Third Era and he probably could fool enough people, and confuse honest scholars as well...
(Whenever Tar-Meena talks about who wrote what and when, tellingly, she tends to use words like "supposedly" and the like - Mankar "supposedly" leading the Mythic Dawn, for instance. I don't think she actually has reliable sources, and she knows that.)