Their doing that is pretty much no different than what many American Historians have done with US history. The PDF I read does not seem to be biased beyond the normal amount on an issue people are interested in doing normally. So I accept what was written until something substantial proves it wrong.
With America, it’s propaganda to make America look much more powerful than it’s. Heck, it’s 2022 & most Americans still believe they were the ones who defeated Nazi Germany but they won’t tell them that America financed the rise of the Nazis & without America’s money - Hitler wouldn’t have been that powerful.
With Nigerians, it’s just stupid tribalism. All the ethnic groups are unique & great in their own way - no need to rewrite history to soothe your ego. Out of all Nigerian groups - I prefer reading Hausa historians. They’re the only ones with no ego & their version of history is well documented cos they have proper written documentation of their history since they were part of Mali/Songhai empires.