If you remember the debate, every time Ryan made a point Biden would start laughing and get the audience to laugh, which would end the discussion. Ryan was not very good but at least he walked back 90% of Biden’s positions, he just couldn’t counter the laughing fool.
Who won a debate is a very subjective question. Ryan "won" if what you wanted was a debate on substance and policy, but he lost on optics, because Biden just laughed at everything he said, offered nothing himself, and just treated him like a child, which is what the audience picked up on.
When I see the Biden of today, I look back to that debate and wonder if that was not a debate strategy to counter Ryan, but that that's about all he could offer even then. We would have seen his senility even back then if he had tried harder.