I really like the theory too. Tbh, it's one if the best I have heard and i am surprised the matter of the fight has not garnered more attention and discussion. Hashirama willingly giving madara his own Yang (giggles), a gift of his own flesh and power, to spare his life out of the reoccured manifestation and travelled path of the younger son's love. It doesn't make tobi a liar but it does show the extent that will be taken to deny the shame and distort the truth.
I like this theory because it so snuggly accepts naruto's prediction, that if he and sasuke fight then they both will die. I think that simple claim and recognition by naruto offers more insight into hashirama vs madara at vote than any history lesson given by tobi.
Hashirama loved madara and to kill him would have been impossible. Madara stole nothing, hashirama must have freely gave madara his power.