It says that Hashirama at some point gained control of the 9-tails in the battle.
It follows that up by saying that Mito sealed it into herself so that she could better aid Hashirama.
So according to the manga there are two options, either of which give Hashirama the advantage.
1. Hashirama gained control of the 9-tails and fought with it against Madara.
2. Hashirama gained control of the 9-tails and Mito (a newly formed jinchurikki) helped fight Madara.
Just think about it, Madara and Hashirama were equals when Madara had just the MS, am I right?
Then Madara gets the EMS, which gives him a new power and restores his eyesight.
Hashirama gets no power boost.
So how can they still be equal?
It doesn't make any sense, and that is why they added the 9-tails being controlled by Hashirama into it.
At first it was a 2 on 1 battle in Madara's favor. Then it switched. And still Madara didn't die, but only became stronger.