What makes you think they weren't talking about Jiraiya at first, then Naruto, then back to Jiraiya? Itachi mentioned everyone have a weakness after Kisame talked about both of them pale in comparison and we see them using Jiraiya's weakness of Hot ladies. Also do you really think they were talking about Naruto being that much of a challenge when Kisame easily absorbed Naruto's chakra and was about to capture him by cutting off his leg yet this goes against what both Itachi and Kisame said.
Also I guess you forgot the fact that Itachi has the MS which allows him to control Kurama so saying they would all be killed or seriously injured makes no senses since they are both not dumb enough to think a Genin will be in full control over their Bijuu. So either Itachi plus backup will all die or be injured against a Bijuu that Itachi can control or they're talking about Jiraiya who did fight multiple jounin-kage level ninjas (Each Pein body was beating Jounin level ninjas no problem) and each had a different power but had shared vision so fought in perfect synch.
