how isnt he an uchiha when bee came after the gedo mazo he summoned a barrier called uchiha kaenjin a uchiha technique witch basically confirmed he is uchiha that wouldn't make sense for him to summon such an advanced uchiha technique if he wasent
What do we know about Tobi,
One he has Obito's Sharingan, and based on the manga he cannot deactivate it.
Two he has Senju DNA, he said it himself.
He knows Izanagi, but thinks it requires Senju DNA, Itachi disproves this unless tobi means if your a none uchiha the only way to activate izanagi is with Senju DNA, a;so hints at him not knowing that Uchihas themselves can use it. If you say Madara said he knows tobi then your mistaken tobi knows madara. What madara said only reflected that he knew Nagato and another person. Tobi could have easily learned of Madara's plan and used it to his own advantage and Madara seems to have no Interest in Infinite Tsukuyomi.