Ujm,
I’m getting my information from the writings of Ethiopian Jews discussing their tradition of their history. When Christianity took over Ethiopia in the 4th Century (CE), the Beta Israel refused to convert and were involved in various wars with the new Christian Kingdom.
This persecution and refusal to convert repeated itself from that time through the 19th century (just like in Europe). From about the year 1400 onward there is a lot documented persecution by the Ethiopian empire (I would post links if it were allowed). The Christian Ethiopian empire conducted forced baptisms, massacres, forced Jews into slavery, and forbade Jews from owning land at various times. This is quote from the Encyclopedia Britannica:
I would recommend the book From Sinai to Ethiopia by Rabbi Sharon Shalom. In their he discusses how the Ethiopian Jews had their seforim destroyed and many were lost to history. He compares Ethiopian Jewish practice with halacha in numerous areas.
I’m not aware of any group in ancient times that steadfastly claimed they were Jewish for even hundreds of years to the point of being willing to be slaughtered or sold into slavery rather than convert to Christianity. What group, ancient or modern, can you name?
I’m not saying that all of the Ethiopian non-Jewish population has Jewish genes. I’m arguing that it is likely that the people closest to them geographically, whom the studies compared them to, had a large mixture of Jewish genes. That doesn’t mean their Jewish, just that they had some Jewish ancestors. Halacha doesn’t follow genes.