To be clear, that paper is not arguing Jewish ethnic groups were intermixing with European locals since prehistoric times, it is arguing that the Ashkenazi Jews

To be clear, that paper is not arguing Jewish ethnic groups were intermixing with European locals since prehistoric times, it is arguing that the Ashkenazi Jews (or perhaps it would be better to say: the proto-Asheknazi Jews) assimilated lots of European converts with ancestry in Europe dating back to prehistoric times.