I mean, how could we imagine that N'Zoth was referring to the same threat Janitor was talking about... when this threat would be alluded to for the first time 2 years after N'Zoth raid?
There was literally no reason for us to assume N'Zoth was referring to this new threat, instead of simply referring to the Janitor himself.
This does feel like a revelation that came out of nowhere, nevertheless the asspull effects of this revelation are mitigated by the fact that N'Zoth's statement was so ambiguous that I can believe he was referring to the new threat.
Even though, when N'Zoth raid came out, we literally knew nothing about this "new threat" the Janitor was talking about... since Danuser hadn't made that up yet. And so, the easiest conclusion with the means we had was that N'Zoth was simply referring to the events of Shadowlands.
Now we know that N'Zoth was not only aware of the Janitor's plan, but also of the same threat that Death was concerned about. Which adds a new layer of complexity to the situation.
So your argument is flawed. If your argument is that Danuser actually set that threat first in the Ny'alotha raid, then he did a terrible job, since pretty much everyone assumed N'Zoth was simply referring to Shadowlands, not to something beyond that. As we didn't have the concrete means to speculate beyond that.