I think he's effectively soldering the steel together with copper rather than a more traditional tin/silver/lead composition using the borax as a flux. The heat of the furnace would be well above the melting point of copper, but bellow that of steel. You can forge a mix of copper and steel, it is sometimes seen in Damascus knives or Mokume-gane etc and mainly used for decorative effect. Probably tricky to get right and I don't think it would do any favors for the final strength of the piece, I haven't tried it myself (being a rank amateur smith at best).