The so-called Icarus Project was the human Imperium's own controllable Abominable Intelligence!
As an essential product of the interstellar age, superintelligent AI had always held a high standing in countless works of science fiction. Whether it took the form of conventional machinery, psychic powers, magic, or flesh-tech, every civilization inevitably developed something akin to intelligent AI at some point.
A person's energy was limited. Once an empire grew beyond a single star system and spread across the Galaxy, establishing a firm foothold in the universe, it inevitably faced one crucial problem: how to allocate resources, or, in other words, how to manage the empire.
Before the Dark Age of Technology, during the period that could truly be called humanity's Dark Age of Technology, humanity created the Stone Men. These precise and infallible Synthetics helped manage the ever-expanding Human Federation.
Clearly, after the Stone Men came into being, the Human Federation's development exploded at an exponential rate. Then the Stone Men created the Iron Men as auxiliary units, pushing that momentum to its peak. In the end, alongside the Aeldari Empire, they became true Overlords of the Galaxy.
That was also why, during the Great Crusade, the Imperium always found humans on the worlds it reclaimed. Humanity had colonized the entire Galaxy long ago. According to the Emperor, that old bastard, some humans had even ventured beyond the Galaxy and colonized extragalactic worlds.
Just look at Guilliman. Without AI assistance, that old workhorse was practically working himself to death—and that was with the human Imperium having lost half its territory and imposing only the bare minimum of oversight on the Imperium in the Dark Side.
The Imperium's administrative system ran on parchment. To put it bluntly, it was a miracle it had lasted this long. And they had Hogg to thank