It could have been written in books doesn't exactly have to mean someone had it. Again nothings proven until it can defend against more then a paper bomb and so far in the manga it hasn't. So yeah it's an exaggeration nothing more.
Databook says "Yata deflects anything"
Zetsu says "Yata mirror can deflect anything"
Orochimaru was searching for it . So , it must be great .
Hagoromo created it , so it must be too OP .
Strongest lightning attack (Kirin) was easily deflected by Yata .
And , if you watch fillers , it could deflect a jubi's tailed beast bomb .