The full conversation
___
DeSaad: Have you finished the conquest? [this being the thousands of planets he owes to Darkseid]
Steppenwolf: Not yet, DeSaad.
DeSaad: Then why do you summon me?
Steppenwolf: I bring news. Before mighty Darkseid came to the throne [omega force], he searched the universe for the ultimate weapon; The Anti-Life Equation. The key to controlling all life and all will throughout the multiverse. He found it hidden on a primitive planet, but before... [he could harness its powers]
DeSaad: The story of the defiance is well-known.
Steppenwolf: I have found the primitive planet. The world that fought back. It is Earth. The Anti-Life Equation is carved into the surface of this very world.
DeSaad: Are you certain?
Steppenwolf: I have seen it. I have looked with my own eyes... of Anti-Life.
___
Ok, this makes no sense. Uxas was looking for the ALE (he searched the universe for the ultimate weapon...He found it hidden on a primitive planet).
He presumably attacked Earth because he was already on a war path (The world that fought back implies all others worlds did not fight back, meaning Earth was one of many conquests to find ALE and/or create more parademons).
How he came to learn the ALE was on Earth is not explained, how he knew he had picked the right planet is not explained. He seemingly figured it out during the invasion, the first time he stepped foot on Earth. But he objectively knew ALE was on Earth. (He found it hidden on a primitive planet)
He is bested by the champions of Earth, a tale that would be given a proper title, and would be easily remembered for thousands of years (The story of the defiance is well-known.)
To suspend disbelief, we have to accept that no one in Darkseid's command remembered the location of Earth (where the ALE was located, Darkseid's entire driving force), no one bothered to keep tabs on Earth (again, the planet with the ALE), and no one bothered to ask if they should return to Earth once Uxas became Darkseid.
So the narrative tells you "Darkseid found the ALE, but he lost and forgot where Earth was, and which planet Earth was all together".
It's as if Thanos knew he wanted the Infinity Stones, found the locations of each one, got bested by the 1,000,000 BC Avengers, and then proceeded to forget where the Stones were and that they could be found again by way of retracing his steps.
It's as if Vader learned Luke was on Tattooine, fought Obi-Wan and lost, returned to the Death Star, and then simply never said anything to anyone about him having a child under Obi-Wan's care, and he also forgets it was on Tattooine.