But that's exactly what it says. :|
Nagato was able to use a power Obito, even with his Senju implants, couldn't.
Nice fanfic, handling dojutsu = strong body
There are several reasons like Nagato having the eys from a young age being able to adjust over that period of time. Or the fact that a reject fodder zetsu clone(goo body) is not a senju body(Hashirama).
Like I said the only canon fact is strong lifeforce for the Uzumaki clan.
Edit: Obito was also fighting, Kakashi, Guy 2 Jins, while controlling the 6 paths of pain, all the tailed beasts and the mezo. His power was divided between that and maintaining control of the eye and his actions. With teying to control both eyes and fighting he'd lose. And if you include Obito as a Jyubi Jin then the effort to maintain control jumps many tiers as Obito had to also contend with the Jyubi and alliance. Nagato had no such issues with his use of the Rinnegan.