He said that his power was reduced over the millennia, which it did. Afterwards he absorbed a couple artifacts and said that he was restored; then he absorbed the power of Mordru and the time trapper (whether it was a controller or the true TT is up for debate because of TT's ambiguous origins) both of whom are above him.
Also, if you look at how regular DS was portrayed in the New Gods comics you'll realize that it doesn't make sense for him to possess such power. DS was searching for the secrets of the ALE which he knew were locked away in the minds of 6 humans; if a weakened DS is capable of controlling billions of super powered beings, don't you think he could just telepathically scan and extract said secrets from the human population? Yet, in the entire Forever People/New Gods series, which focused on that plot, he was unable to do so.
Yeah, Darkseid's baseline psionic abilities have never come close to what we saw in the GDS.
Telepathically controlling 3 billion Daxamites, and maintaining that control even after a lengthy fight with Orion AND Highfather-amped Superman/Supergirl... That's ridiculous.