What?
The Elyah Rabah seems to discount the story from the Sefer Chasidim (thereby undermining a potential basis the halachic validity of DNA testing) based on the story of Rebbi Banah in Bava Basra. The logic was that if the test of the Sefer Chasidim (see whether the father's bone absorbs the blood of the potential son) worked/was halachically valid, Rebbi Banah should have just done that test instead.
But that logic assumes that the blood of the mamzerim would not have been absorbed by the father's bone. Why wouldn't their blood be absorbed as well? In the story of the Sefer Chasidim, the blood that was rejected was the servant's who was not related to the father at all.