1. You can't prove a statement by bringing up that statement again. Oodama Rasengan have never been shown to be mountain busters, so the Databook is contradicted, making it false.
2. Madara wasn't shown after that attack, so we don't know what shape his Susano'o was in afterwards. Seeing as how it tanked a TBB from 100% Kurama only 1 stage later, I'm inclined to believe Gai did nothing but push him away.
3. Sage Mode is Juubito's weakness. Since Sasuke did not have Senjutsu chakra, Juubito's skin was basically invulnerable to Sasuke's attacks, meaning Susano'o hardness has no meaning.
If you take Kakuzu's Earth Spear's Lightning weakness as an example, even a Fuuton attack of the same level as Kakashi's Chidori would not penetrate it. That does not mean that the Chidori is any stronger than the Fuuton attack, just that the Earth Spear is specifically weak to the Chidori.