I guess Darkseid attacked Earth 3 and they took him down like Main Earth. But to his surprise, I guess they took over this BoomTubes and are now able to travel between Universes, with the exception of Earth 2, b/c Batman's gadget took down Steppenwolf's towers.
I am however extremely curious as to how they were able to blindside Xanadu - what magics are they packing?
And get over the Batman thing. That's not Bruce, it's Grayson.