Cobo is the go-to source for what is known of the Inca's history, unfortunately. While he apparently did the best he knew how to he was very limited in his resources. This is very much an approved history of the ruling family, as the puppet Inca installed by the Spanish still held life and death power and stories reflecting poorly on his ancestors would have been left out. Cobo also made the same mistake that most modern historians make, thinking that there was a single authoritative accounting that could somehow be extracted from an extremely flexible oral history tradition. This means that Cobo's narrative is different from Garsilaso's, which in turn is different from Guaman Poma's, which is different from ...
Having noted this, and accepting the fact that it was written down from the viewpoint of an European barbarian, Cobo did an admirable job of recounting the official history in a clear and logical progression. Just don't make the mistake of thinking it's the *only* version or actual history.