You've missed something that I can't see :-(

If I export your file as pdf - I get pure black text :-)

If I export my file having ticked "Always Overprint Black" and ticked "Print all text as shapes" (you had it ticked) - text is 4 colour :-(

I even cut and past just one para into a new doc (incase my images were interfereing) and it still outputs as 4 colour.

Can't see any other setting that you've made that is non default (oh apart from Commercial printing in the pdf export).

What is the last magic step?

Turan