While your proof is skeptical at best I agree. Itachi was on his death bed an nearly blind and he was capable of fighting and not losing to sasuke but his illness killed him in the end.
Best indication of Itachi's stamina is his Part I self, where we can assume his illness hadn't taken full effect. There, he used three Mangekyo techniques, a Suiton, a Kage Bunshin (which splits chakra in half), a standard Genjutsu, and still wasn't on his knees gasping for breath like the battle in Part II.
He outlasted Hebi Sasuke and did that while using more chakra-taxing techniques and the moves in his arsenal require greater skill. People do honestly overexaggerate his stamina. For a man on his death bed with a fatal illness, he didn't do bad at all.
You don't need large chakra reserves to fight like that.
Being at the pinnacle of chakra control is what allowed Itachi to use spam those techniques on his deathbed. And even after that, he still outlasted Sasuke.