It isn't. That is just one (of several) rumors for its origin that your research on it comes up with in Legion, the actual nature of the blade is completely unconfirmed.
It being able to imprison the sentience of an Old God is exactly why he'd want it on hand rather than in Xal'atath's possession (and why having it was worth freeing her). The blade is extremely resilient, Natalie Seline intended to destroy it when she went looking for it, but realized it was impossible as soon as she touched it. It's quite possible N'zoth doesn't have the means to destroy it, particularly if it was designed to contain someone like him.
If it takes Azeroth dying for us to get a revamp, sorry honey you gotta go.
Are we even sure her dying would be that bad for us? Maybe it's like when a chicken dies in the egg. Just kinda stays in there. Maybe it would be bad over time but I'm not convinced it would be bad immediately.
(No world revamps, please. Not again. Things which take this many resources should be added on top of other things, not replace them. Thanks.)
I think wide-scale revamp 2.0 is up there with "your player dies lol" and "player race entirely goes evil/gets deleted from the game" in the Never, Ever, EVER Tier of happening.
EDIT: And no, smartass, Argus fight doesn't count.
Well could mean nothing but I always use this: https us.forums.blizzard com/en/wow/c/in-development/815-PTR-General Link to get to the forums and it's also away, but maybe it's down since last Thursday.
Feel like Thursday more likely for PTR release than tomorrow, since tomorrow is the raid release of Crucible of Storms. Will be happy to be wrong here though
I think it'll be today so they can officially focus on Crucible of Storms for the week while fansites cover the stuff they won't.