Unfortunately it's not. I made a mistake here (guess I mixed things in my head). I meant the character 'z' but 0x79 actually a 'y'. I will fix the issues later today.

How is it technically correct?

If you define "the z of x" as the last element of x, then it is correct. Whether such a definition is reasonable (especially when x already contains an element specifically named "z") is left as an exercise to the reader.

EDIT: I suppose this doesn't include 0x7A-0x7F either so it's not even technically correct under that definition.

Base 10 society strikes again! This happens to me too, it's hard to think of [0-9] being only 62.5% of base 16.