I don't think that the human inhabitants of Sera came from Earth. Their technology honestly comes off as way too primitive for that to be so. If you have the technology to travel to and inhabit a planet within another system, why would you be fighting enemies with machine guns and grenades? Even our armies have more advanced military tech. If Serans really are from Earth their technology should be advanced enough to just completely destroy the Locusts.
I think Sera is supposed to just be an alternate take on Earth; to have many similarities to our planet - enough to make the setting seem familiar - but its own history.
I think Sera is supposed to just be an alternate take on Earth; to have many similarities to our planet - enough to make the setting seem familiar - but its own history.