Huh...nice thread. Though there does exist Noddy's argument which tries exploits Kishi's (arguable) mistake; that two absolute invincible items can't exist at the same time, for their very existence contradicts said statement(s) about them, which could lead to their hype, respectively, being invalidated. Though in my eyes, Itachi's Yata Mirror is superior to the aforementioned "invincible" items, solely because Orochimaru was also in search of them, and Itachi's Mirror also has considerably suitable feats. Moreover, it's not a stretch to say that Itachi's defence really is absolute as Madara's essence (Black Zetsu) said that Itachi was invincible with those two legendary weapons in hand, Madara being an Uchiha possibly knew of these weapons as they seem inter-usable, with Susano'o, and without. This hints that Madara could have been in search of them; further suggesting that Itachi's Mirror really is the perfect defence.
In conclusion, it's likely Itachi's defence really is absolute, though one can't be absolutely sure.