Board Thread:Theories and Speculation/@comment-32595957-20170716175646/@comment-5085906-20170716181657

It's a solid theory, which you debunked in your own notes. Sakumo was said to be on par with the The Legendary Sannin, yes, and the Legendary Sannin were Jiraiya, Tsunade and Orochimaru. So if he operated at the same time as them and his prowess was repeatedly compared to the Sannin then there's no way he could be Jiraiya since Jiraiya was one of the Sannin. I butchered my point, but I hope it got across to you.

There's also the fact that Kakashi met Sakumo in Limbo when he got killed by Deva Path and he definitely was not Jiraiya, as evidenced by the lack of tits-related metaphors in their brief conversation.