As always, the userbase being mental dwarfs.
Itachi himself stated that under any circumstances, neither he nor Sasuke should use skills that could be deadly to Kabuto because he did not want to kill him. He didn't even use Version 2 Susano'o, let alone his best version.
Itachi wins this via Totsuka Blade.
Under the circumstances that Itachi was found in of not trying to kill Kabutomaru, even Orochimaru would hurt him like Kabuto did if he's invulnerable to Genjutsu. We all know what happened to an Itachi that was about to die with just the sole intent of killing: he still wrecked Orochimaru with a technique that he did not use against Kabutomaru because of obvious reasons.