IIRC there are additional steps (a special file?) that are Apple specific to use embedded fonts
Nothing jumped out at me.
Weird: I opened the file with the Calibre Viewer I see the black-letter.
I open in Sigil 7.3 (on XP), I see a Sans face
I tried to validate the CSS to CSS 2.1
lots of not supported in 2.1 which may drive other devices off the deep end .