From all the info I've gathered the Archimonde we fight in WoD is the same Archimonde that tried to hug the World Tree in WC3 which basically means demons can travel between alternate realities.
Alright, but we have(had) two Velens. This other alternate Velen that sacrificed himself also ruled alongside an archie and a kil'jaeden in his AU. So where did those chums go... ?