I'm thinking the centaurs would have driven the humans westward in earlier times, since they are pretty much unmatched in the open. I've read your history and if the centaurs were in those lands at the time the Light Ones were passing through, they would have done what they do best: outrun them at every turn, inflict damage if they were being pursued and generally avoid pitched battle while making the stay of the enemy as inconvenient as possible, depriving them of supplies, sleep and so forth.

Anything centaurs can't fight they will simply avoid.
However, having read your history raised an interesting idea: the Light Ones traveled through their lands once, not stopping and settled lands far beyond, to the south-west. That would have taken them some time before the conqueror Light Ones decided to make a return trip the same way they came.
This time though, they would have been facing not disparate tribes, but a single, massive horde of the entire collected centaur kind. They could have been the reason the centaur tribal confederacy would have been founded. Now depending on what you would agree upon, we can range from the Light Ones being simply routed, taught a lesson or obliterated entirely.
Do note that centaur combatants are hunters by nature and they would have lured them in through ruses into traps.

As far as my territory is concerned, I was facing a difficult dilemma. Not only are my people nomadic, meaning they need plenty of space, but they're also tribal, with an untold number of tribes present that are constantly moving across these lands with the seasons and weather patterns. Population density is extremely low so none of this land is actually settled or even routinely secured.
There's nothing stopping people from just going in there. However, the area mainly denotes a designation of land centaurs see as their own and they will kill trespassers on sight if they find them. And in open field, it's difficult to hide for long, particularly if the ones you're hiding from have every physical advantage on that terrain.
So you may go and squat there for a while and live in fear of discovery every waking moment when you lay your head down and wonder whether this will be the last time you woke up. With tribes continually migrating, settling here without the centaurs' consent is practically impossible.

Let me know what your thoughts are on this :)