Shihab al-Umari, an Arab historian of the 14th century, wrote much of what's know of Emperor of Ethiopia Amda Seyon I campaigns against the Muslim States, namely Awfat, Dawaru, Arababni, Hadya, Sharkha, Bali and Dara.
Prof. Taddesse Tamrat cites al-Umari  (emphasis mine):
"their weapons of war are the bow with arrows resembling the nussab; swords, spears and lances. Some warriors fight with swords and with narrow and long shields. But their principal weapon is the spear which resembles a long lance. There are some [warriors] who fling darts which are [similar to] short arrows, with a long bow resembling a cross-bow."
Now, the way the phrase is worded makes it resembles the description of a solenarion, that is, an arrow-guide used in conjunction with a bow that allows one to shoot really short arrows, seemingly to really long range. This device, used while attached in some places and detached in others, is known under many names among Byzantines ("solenarion", σωληνάριον), Turks ("majra" or "nawak"), Chinese ("tongjian", 筒箭) and Korean ("tong-ah", 통아) sources. I've read similar accounts on ancient Christian Nubia as well, sometimes mentioning poisoned darts.
Is there solid evidence of the use of it among Nubian and or Ethiopian medieval militaries? Perhaps al-Umari or other historian under the Mamluks could have mentioned it in other works.
Alternatively, is the presence of the cross-bow attested in Ethiopia during Amda Seyon I times?
 Tamrat, T. (1968). Church and state in Ethiopia, 1270-1527 (Doctoral dissertation, SOAS, University of London).