From what I understand, there's no actual evidence that Mills was Jack the Stripper. At some point, the police apparently revealed that the prime suspect in the case had been a middle-aged ex-boxer who committed suicide. Mills fit the description for that, so some author speculated that he was the killer.
This is untrue the police made no such statement,and the prime suspect was a former detective. Mills was gay but not a murderer, James Morton 's book Fighters demolishes this theory.