Darkseid's henchmen were interrupted from their personal battle at the end of "Alive!", but they were not summoned into the battle, which would have made sense since otherwise there seems to be no point that they were readied to do so. See more
Okay, let's get these people locked up. Sounds like we've got a fight on our hands.
If you think you're locking us up while the whole world is under attack, you got two fights on your hands.
[the Leaguers and Legionnaires stare each other down, ready to fight
She's right. We'll need all the bodies we can throw at this.
Oh, come on! It's Lex Flippin' Luthor! Why should we trust him?
Hey, it's our world too!
Let's be clear about this: we're not here to help you save the world, you're here to help...