Well I don't think that it is not just coincidence that only the Uzamaki Clan Shinobi are the only clan who are able to supress and house Kurama's chakra. It is not also coincidence that the same SEAL that Naruto used on Kurama to seal him was recognized as a seal from Rikudo Sennin. Now how is that possible is the Uzamaki came from the Senju, the same Senju Clan that was formed after his death by his Youngest Son who got the body of his father.
It hasn't been fully explained as to how Rikudo Sennin got the Rinnegan but it a fact that if he would have had it before he fought the Juubi then it wouldn't have been a fight since he would have been able to just control the Juubi with the Rinnegan. So oI say that he got the Rinnegan from the Juubi by being the Juubi's Jinchuriki.
Getting back to the subject. There are too many supposed coincidences that make it hard for anyone to argue that he would have been an Uzamaki. He used a very powerful sealing technique so that right there should have been a red flag that he either originated from the Uzamaki Clan or formed it himself. The Uzamaki Clan are the only clan known for being the most powerful Fuinjutsu Masters. Hell it should even be more known to be the truth now after they came back from the Uzamaki Temple. There were the masks on the walll and I am sure that the one that Orochimaru picked up was the only one that worked. I am positive that each one of those masks had different abilities or sealing techs probably more powerful than the Death God.