Theory: The first Ionian humans were Icathians
Taken from the Universe page: "The vastaya’s origins can be traced back to a hidden corner of Ionia where a group of humans fled in escape of the Great Void War"
Now, we know that the Icathians were annihilated by the void, but what if there were some who survived? And so the remaining population fled. Think about it, even the names are similar. Icathia, Ionia...
The Vasthayshai’rei were very welcoming of the refugees as the universe points out, and the refugees didn't pillage and destroy the Vasthayshai’rei home land like a lot of other humans would have, maybe because Icathian's knew how it was to have their home taken away from them? And we know Icathian's were very accepting of different types of species: Especially because it is hinted that Icathia was very inclusive of all races: Here and especially here. So it stands to reason, the Icathian's weren't bothered in communing and developing relationships with these shapeshifters, and the Vasthayshai’rei felt comfortable with the Icathians.
Now... How the hell did the Icathians make it all the way up there?! Well that's simple, I touched on something similar to this in another post, but Icathian's would've had a very good ocean port with international trade, as per why Shurima considered their state so viable for imperialising that they would send 12 GOD WARRIORS. 12 OF THEM. Lol.
So we've established Icathian's were good with ships, they wouldn't have trouble at all navigating through the ocean, WHY Ionia? Why all the way up there and not south or east? Why north? Well, Icathian's were probably very aware of established societies in the other contents and maybe they knew other societies wouldn't really like a rebel population of Shurima coming to stay, or Icathian's were wary of other humans because of the Shurimans so they decided they would find somewhere where humans didn't love. Or, simply, by chance.
About 100 - 150 generations have passed in the estimated 3,500 years since Icathia.
All of these champions are descendant from Icathia, how crazy?

And then we got the old bois:

's lore states that the time of Avarossa, Serlyda and Lissandra was long before Shurima and by extension the Fall of Icathia. The Watchers now being the Masters of the Void means that the 'Great Void War' is in reference to the war between the Watchers and the Iceborn, when the Watchers first invaded the Freljord in the time of the 3 sisters.
The transition from Freljord to Ionia is also a common journey we see both
and
do, opposed to straight from Shurima to Ionia.