That's looking at it backwards---this is a non parametric approach. The author didn't assume a priori that a point with a z score greater than N was anamolous,

That's looking at it backwards---this is a non parametric approach. The author didn't assume a priori that a point with a z score greater than N was anamolous, for some fixed N. Instead, they determined N by looking at the past data. (Contrast with the parametric approach of assuming normality and concluding that any point lying >3 sd from the mean is an anamoly due to properties of the gaussian distribution.)