Couldn’t “who was called Christ” be a copyist’s marginal note that slipped into the text? And James was a common name, so this may be a different James.

The interpolation theory has been examined and rejected by nearly all Josephus specialists. The phrasing is exactly how Josephus disambiguates common names elsewhere, it is neutral rather than reverential (called Christ, not who was the Christ), and Origen quotes the passage in the third century, before any plausible Christian editing of Josephus. On the different-James point: the identifier only works if readers already knew of a Jesus called Christ, which is itself the evidence this exhibit points to.