Ujm,
That’s just wrong. The Old Testament is a translation of Tanach. It isn’t heretical the way the New Testament is. The Alexandrian Jewish community used the Greek translation for generations.
Why is your standard “clearly documented” over oral testimony? In halacha we accept oral testimony over documents. The Torah sh’bal peh was oral for over a thousand years.
There is plenty of documentation, however, of the group’s history and identification from Christian and Muslim groups that interacted with them (including descriptions of their persecution, forced convervions, burning of seforim, etc.). Additionally, there is the writings of Eldad HaDani, Benjamin of Tudela, the Radbaz, and some secular Jews that visited them in the late 19th and early 20th centuries. There are also letters that the Beta Israel community sent to Jewish communities in Europe and Jerusalem in the mid-19th century describing their communities and their plight.
I’ve seen the Radbaz and one of the 19th century letters. For the rest, I am relying on other people’s articles citing these sources.