I reckon you're over reading it. Mashima didn't even think of Susano back 10 years ago when he wrote those lines. From my perspective, have read it as it goes, instead of reading in bulk, I reckon what Itachi meant then was based off their reputations. A Sanin >>>> mist swordsmen and Uchiha.
Also, based off your theory, you said Akatsuki consist of 6 members at that time, but weren't all the members present when Jiraiya told Kakashi about Akatsuki? Did he not say that there were 9 members at that time (+Oro)?
As for Itachi knowing about Jiraiya's abilities, that I doubt even more. I mean even Pain wasn't that knowledgable about Sage Mode and he studied right under him. If Jiraiya hadn't lost his arm due to his carelessness, who knows, perhaps he may have won against Pain, eventually.