When you wrote this the Black Empire was still seeping through the timeways. Hell, traveling back to the past prompts N’zoth to point out that he’s going to know you later.
Nzoth let himself go for the same reason he corrupted the Bronze and let Xalatath fall into the hands of priests. Time is nonlinear in Azeroth and, as the Ebon Blade put it, “Death is not an end in this world.”