The revelation that the Runeterra map we know is just a small part of the whole of Runeterra is a cool one as it allows Riot to, if they want, introduce more into the lore (after they finish the updates obviously). It's similar to how Thedas of Dragon Age exists. In that world, which has a defined map, all the events are contains within what appears to be a few nations over the size of say Western Europe. Yet the lore and history goes back 8000 years, all contained in this one area. They (the characters) are seemingly locked in this map and haven't ventured out of it. According to it's history, Humans aren't even from Thedas, neither are Qunari, but the history is so old that most don't know where they came from.
What's sad is that the 'world' Darkin were from was not just another part of Runeterra, they could have easily and quite uniquely made it so that Darkin were from another part of Runeterra and the Five remaining were trapped on the known area we know. That would have been a great retcon, not they were just men and they are the reason they are murdering scum.