Because it doesn't contradict manga canon. The 4th databook has information that didn't happen in the canon manga but happened in fillers, yet because it doesn't contradict the manga, it can be included.
How does it not contradict with the manga when Naruto is performing jutsu by twisting his fingers, Gai is walking despite his legs being broken or shattered during Madara vs. Gai, and the list goes on?
I haven't read that but those are minor details that can be overlooked. Didn't the manga say that Hagoromo sealed the Juubi by himself on his death bed and created the Moon as a result but then it was stated him and Hamura used their seals to seal away Kaguya and form the Moon that way.
That would make sense or it might be a translation error.
