You make it sound like the sage came out of nowhere and defeated the Juubi. He may have been a god, but the manga stated that he became god once he sealed the Juubi inside of himself...
I didn't I said that he doesn't need to be in a clan, he could as well been a private person. It was as well stated by J-man and Wiki that Rikudou Sennin was a Monk/Priest. So when I say a private person doesn't mean that he would have been a fodder or anything like that. Just that he is Neutral and isn't in any kind of clans, and probably he did pretty much just pop up and defeated Juubi. It's not really that we got any details about his life or Juubi's life.
So I think we should all just wait and see if we get any further details about Rikudou Sennin before trying to say that he is an Uzumaki, I am not saying that it is impossible, but still so far we know about him is that he is the strongest person that was alive, he is the founder of true chakra, he is the true savior of Shinobi world.
Not so much more to say, so he is probably a character that won't be named. It's just something that Kishimoto wanted to back up his plans about how they found the power of using chakra, how war was created and how the Tailed beast was created. Nothing more then that. That is what I believe.
