Hello Rabbi, I know Rav Sherira Gaon wrote the Iggeres on the history of the bringing down of the Oral Torah, especially the Amoriac period. My question is, how come historians rely on this Iggeres, as it is only a single source? I guess the Iggeres isn’t bias, as it is just a collection of chronological accounts, but still.

Rav Shrira Gaon's Iggeret is solidly based upon many many sources, as is apparent to anyone who has gone through rabbinic literature, and as documented in the footnotes of Rav Dr. Binyamin Menashe Levin zt"l in his doctorate which was subsequently published as the scientific edition of the Iggeret. In short, it's not at all a single source, but each line is solidly based on a source, altogether many many sources.