Board Thread:Naruto Discussions/@comment-36006024-20180926134241/@comment-32685487-20181003135724

Six Paths Senjutsu is most likely just Six Paths Chakra and natural energy infused together instead of normal chakra and natural energy. So Hashirama already possesses Sage Mode Mastery so if he was to get Six Paths chakra he would attain SPSM.