There is no proof that itachi is as strong as every one says. In naruto anime itachi said jariaya was more than a match for him. Even though that was a old anime Naruto series is not the type to regret the things they said in old episodes. Also the only person itachi really fought alive was sasuke even though he wasnt at full strenghth and let sasuke win on top of that. This still does not prove much because sasuke was not at that time on really any high level of strength . Sasuke also did not have mygenkyo sharingan. Then Itachi came back as edo tensei itachi and fought naruto for a very brief time and that wasnt enough to prove his true strength also. He then stop nagato but nagato was distracted so this also did not matter. The last battle Itachi had he fought kabuto with sasuke and got cut in half more then once.
and just so you know I am not a itachi hater