Meaningless when Itachi blitzed a 45 year old Uchiha who had 3T activated ALONG with two other 3T adults.A thirty-six year old man.Itachi at the massacre would have destroyed him.
Now compare that to him fighting alongside Shisui of the Mirage: a chain smoker, with one Byakugan, self taught, with taijutsu not even good enough to surpass 16 Palms.
Eye contact.The Tsukuyomi cannot be broken unless someone has the Sharingan and Uchiha KKG, meaning your buddy is toast.
Eye contact.Not only that, but a non-destructive Tsukuyomi killed Izumi so if Itachi wanted to kill that half-breed Hyuga during the time of the massacre, it would have been easy.