I think it is possible that Targon was once a great empire like Shurima and possibly even the most powerful at one time. I also think that Shurimas and Targons high points are much more acient (akin to Acient Egypt, Mesopotamia, Persia, Greece) than Demacias and Noxus (akin to 100 - 1600 AD Europe). So in my head canon, I would think that Targon would interact with the early ancestors of Demacia and Noxus (Similar to Roman Britain 43 AD - 410). I also think that with the fall of Shurima other nations and empires where able to capitalize a expand (posibly Noxus and Demacia).
Just as a final disclaimer, this is all my head canon.