I have wondered myself why the Ethiopian Copts aren't held forward as a shining example of sub-Saharan African Christianity. Almost no one knows of it.
One question though...is the Ethiopian Church Orthodox or Monophysite?
The Ethiopians are monophysites, though it is unclear whether out of conviction or simply out of the historical circumstance of being outside the Empire and stranded with contacts only with the Copts after the rise of Islam.
There were Orthodox churches in Upper Egypt, what is now the Sudan, as testified by icons of St. John Chrysostom, who until recently had been under anathema from the monophysites.