Psukkar- u said its not stated in the manga. But in the manga i believe chapter 500. Kushina tells naruto do u know about the battle between the 1st and madara. Naruto replies yes. She follows by saying DURING that battle the 1st supresses the fox and in order to better aid him, Mito seals the fox inside her. Thats in the manga. So how about u read it over.
yes during the fight hasihrama did take the fox.
U mistrept what was said.
It was never said WHEN the sealing took place.
We know WHY hasihrama got mito to seal it within herself to try and control it for konoha sakes. But when this took place is unknown.
And if you think they sealed it then there and then mito somehow was able to control in a blink in a eye to use against madara ur just fooling urself.
How long did it take naruto to get anywhere near using the fox? 6 years at least.
Important you realise the fox doesnt like shinobi and would rather see his host die. Thats obvious and mentioned by himself why he hasnt help any of his hosts in the past until naruto.
So try and read between the lines you'll realise its unlikely mito was able during a such a epic battle seal a fox with madara there fighting hasihrama.
U think it takes 2 secs to seal the fox or soemthing.