Would be cool if "Solution 9" means it's the 9th solution they thought of. Maybe Solution 9 was the one that involved transporting your whole city into a different shard (which happened to land on the source), but it also meant leaving the rest of your world behind. Could've been their last resort, after all the other solutions failed. They could've come from any shard, not necessarily shard 9. Maybe even from the most ancient shard (the first that was rejoined, whichever that one was) which would explain why their alphabet is so close to the ancient's and also would explain the high tech: they've lived for longer than any other civilization in the Source.

Honestly as cool as this theory sounds, it almost sounds too... obvious? simple? I don't know, I'm not fully buying it but as of right now that's the most interesting thing that I can think of. I'm ready to be surprised though!