Negus, the Ge'ez word for a king, begins with this letter, and the imperial title negusa negast, king of kings, doubles it. The title was borne by the rulers of Ethiopia until the monarchy ended in 1974, and the style has a long history behind it: the kings of Aksum used a comparable formula in their inscriptions, and the phrase is the same construction as the king of kings of the Persian and biblical record.
The claim the title carried in Ethiopia was more specific than a statement about rank. The Kebra Nagast, the fourteenth century work whose title means the glory of kings, tells the descent of the Ethiopian royal line from Solomon and the queen of Sheba through their son Menelik, and makes the removal of the Ark to Aksum part of that account. The dynasty ruling from the thirteenth century presented itself as the restoration of that Solomonic line, and the title sat inside the claim.
The atlas records the Kebra Nagast as a work with a date and a purpose rather than as a chronicle. It is a national epic composed at a particular moment in Ethiopian political history, and reading it as a record of events is a category error. The letter that opens the royal title is a convenient place to flag it, because the title is the thing the epic exists to justify.