"An Outside Context Problem was the sort of thing most civilizations encountered just once, and which they tended to encounter rather in the same way a sentence encountered a full stop."
- - Iain M. Banks, summing up the end result of Star Trek type societies like The Interex encountering the Imperium quite nicely
The Interex (both the demonym and the empire, used like "the Imperium") were basically what the Tau think they are.
The Interex, probably the closest thing we can glimpse about Human Civilization during the period of enlightenment known as the Dark Age of Technology, was made up of technologically advanced humans, but also alien races like the kinebrach, which were like space gorillas. It is notable to mention that the Interex learned about Chaos from, and were tutored by, the Eldar. Interex ships were bigger and more advanced than those of the Imperium, and the empire also made use of AI and robots (that could additionally be used to augment their troops). While they were very powerful and capable of wiping out hostile xenos, they felt it was better to just downgrade their technology, eliminate space tech and scale the populace down to one planet, surrounded by warning satellites. They were aware of and fought against Chaos, which they called Kaos, unlike the Imperium at the time, which officially denied the existence of daemons and spirits. The Interex language was augmented/accompanied by some kind of music based on universal math called the aria, which made them cool and not prissy. Their warriors could ride on four-legged robots to form a centaur-like warrior that fought with laser arrows that could easily penetrate power armor. Regular Interex warriors could just about take an Astartes in single combat, mostly because the Astartes underestimated them and because the Interex's fighting styles were almost unrecognizable to them. They felt that war was bad though, and this idealism would lead to their replacement by the Imperium as prime galactic power.
In the first Horus Heresy book, Horus meets them and decides to actually negotiate with them, even though he knows the Interex makes use of the robots and AI, which are abolished by the Emperor, and because he feels that the galaxy is too grimdark and the Imperium purges things too often. When it seems that everything is going well, and the Imperium might just not suck if Horus thinks like this, Erebus steals a kinebrach sword called an Anathame, THE MURDER SWORD (it's related to Nurgle), from a war museum called the Hall of Devices, blows the museum up, and starts a war. Horus is scratched, and sulks off to feel bad, while the Interex are mentioned to have been purged in an offhand mention in the next book.
The Interex makes an excellent contrast against the Imperium in several ways. Firstly, there's the comparison between them (or what little we know of them) and the Imperium both pre-Heresy (already showing the rot beneath its gilt facade) and post-Heresy (techno-barbaric theocratic fascist hellhole). More obviously, there's the whole comparison of their own attitude towards Kaos/Chaos and the Imperium's attitude towards the same, which highlights just how useless both the Imperial Truth and the Imperial Cult are.
Especially the latter: for all its fans defend it, the truth is that countless Imperials are lost to Chaos by ignorance or forbiddance-induced curiosity every day. Humans don't know anything about Chaos, or else know just enough to be intrigued, but not enough to know the full dangers. Even the Space Marines, whose brains are much more "muffled" to most human desires and are brainwashed with Imperial propaganda as a part of their creation, go rogue all too often. In comparison, the Interex made the truth about the denizens of the Warp plain and simple to all its people, ensuring every member of their culture knew exactly what the Chaos Gods were after and the dangers they posed. The result of this education? Chaos basically couldn't get it's hooks into any Interex: they were so powerless against them they had to manipulate the Space Marine Legions into doing their dirty work for them. If not for the God-Emperor of Mankind and his totalitarian dreams for his Imperium, perhaps it might have been the Interex who would have led the reunification of humanity.
While some people argue that their lack of militarization would have led to catastrophe if the Tyranids or Necrons were to show up on their doorstep, it is still intriguing to imagine what might have happened if humanity's primary civilization was not based solely on war and xenophobia. Had the Interex been in charge, the far future would not have been quite as grimdark, but almost noblebright. Which just makes the setting of 40k even more grimdark. It's also worth wondering what other Legions might have made of the Interex if they'd come across them before Luna Wolves did. Like, say, the idealistic Ultramarines, or the technophilic Iron Hands. Or even better: The Thousand Sons. That would have been hilarious (from a safe distance, anyway).
But was the Interex truly immune to Chaos?
Let us consider for a moment a few points, first off the Interex had received help from other alien races with some experience on dealing with Chaos, indeed, this is an advantage which may not have been wielded by many human civilizations in the setting.
Then there is the simple fact that the Interex was a post-scarcity economy, this alone means that next to no one would like to attempt to deal with enemy forces out of physical desperation, which is often one of the major reasons why Chaos takes hold in the poverty-stricken worlds of the Imperium, like in real life, the vast majority of people will not feel any interest in joining the enemy, or a strange cult, when things go well.
The Interex was a collection of small territories when compared with the Imperium, so it was far easier for them to manage their population and bring education to the masses while keeping a better control over them, and if this sounds familiar to you, great, you are paying attention.
Finally, Chaos really didn't need to corrupt anyone to take hold over the Interex- all it needed was to use a catspaw to wipe them out on their behalf.
Ultimately it can be recognized that the Interex looked like a possible path for a better future for mankind, but all it takes to fuck it up is one guy willing to take advantage of a bad decision (seriously, why did they put a Chaos artifact in a museum instead of destroying it or hiding it where nobody could steal it?).