To me it would have made more sense to have Scott Free and Barda on earth with their box or just have Cyborg find the one box instead of them hiding a bunch of boxes under some dirt.

Yeah its true the first 15 mins are about the.prehistoric war where Lanterns helped people of earth to defeat Darksheid this is a scene where we will see Darksheid don't know who will voice him.

"Roughly 30,000 years ago, Darkseid attacked the planet Earth. A coalition made up of humans, Amazonians, and Atlanteans managed to fend off the cosmic psycho. Fleeing the planet, he leaves the motherboxes behind, prompting Steppenwolf to retrieve them"
Ugg, is someone really going to narrate that part?

Those i believe were Atlanteans, not vikings.

i think he means that prologue, not the events. the prologue sounds "not so good Al." but maybe the right voice will bring it together.
=w=

As per the Justice league trailer, there are hints that Darkseid/Steppenwolf had invaded earth in the past. You can also see the vikings burying a mother box in the ground.

"Roughly 30,000 years ago, Darkseid attacked the planet Earth. A coalition made up of humans, Amazonians, and Atlanteans managed to fend off the cosmic psycho. Fleeing the planet, he leaves the motherboxes behind, prompting Steppenwolf to retrieve them"
Don't believe that's real for a moment.
"A big ball of wibbly wobbly, timey wimey stuff"
The Tenth Doctor explains all.