Treating Jesus’s existence as a starting point isn’t an argument. She assumes it because everyone in her field assumes it.

A field-wide starting point is not the same as an unexamined assumption. Fredriksen reconstructs Jesus inside Second Temple Judaism using the same tools historians apply to Hillel, Honi the Circle-Drawer, or the Teacher of Righteousness at Qumran, and the reason she can treat his existence as settled is that the sources pass those tests. She is cited to show that the consensus is not an evangelical echo chamber but the working assumption of mainstream historians of Judaism and early Christianity, including ones with no religious commitment.