Its what manga says, implying it could be just hype, so selective reading aint gonna get you far
I completely agree , those Itachitards boys believe in every word of Black Zetsu , they will just end up like Madara.
BZ says in the 1st sentence of the scan , that this sword is
"said to carry sealing powers".
That means that:
1/ BZ didn't saw that blade in action before ,
2/ He is telling us now what someone told him before ,
3/Those sentences can't be taken as mangafacts.
Thread soloed.
In that scan , he is realising that part of what he was told was truth (the 1st sentence).
He also find hard to believe in the last words he was told (the 2nd sentence of the scan) , he starts doubting when he says "and so the legend begins" , you mix truth with hype.
The 1st sentence is truth , the second is hype.
I don't see why I would believe in the guy that talked to BZ about that blade , even though parts of what he said was truth (Totsuka blade seals whom it pierces).
The real manga fact is that this blade has sealed Orochimaru , that's all but that's still huge.