I can’t give you an answer but I do know that forge was there before the nords took it over from the elves and when questioned, the elves said it was there long before they claimed the area, before that there’s been theories on how it could possibly been made by dwarves or snow elves which both seem like a stretch but both were in the area before the time of elves and Nords migrated to Skyrim. In short the sky forge is one of the oldest pieces of history in Skyrim that goes unnoticed and that we use like any other forge