Ujm,
You are just ignoring the evidence you don’t like. They are not “heretofore uknown” (unlike the Lamba), they were known to the broader Jewish community from the Radbaz, Benjamin of Tudela, and Eldad HaDani. (And it was known that there was an a Jewish community in Kush from Tanach). Additionally, there are numerous references to them in Muslim and, especially Christian sources. They had wars, over hundreds of years, with their Christian neighbors. This is isn’t just some oral tradition that doesn’t match up with other evidence. It is an oral tradition that fits in with the rest of the documentation we have.
We have a letter that was sent by a leader of the Beta Israel in 1850 (or so), in Hebrew, to Jerusalem asking for help to emigrate to E”Y. That’s 170 years ago.