As others said, not too shocking that the truth is "this is planet Earth". So now my speculation is that there are at least three "out of time" humans.
First, Shiromine ofc. He hails from our present time the 20xx time period.
Next is Deitriech himself. I suspect that his deep hatred of inhumans is because he is from the time period of the invasion. To him, it is inconceivable that humans could get along with the "monsters" that caused the collapse of human civilization.
That brings us to the newest antagonist, Seed Eight. I am guessing he is a person from just before the invasion, when humanity was at its peak. That is how he can invent so much technology, to him it is just recreating common tech from his own time. It also explains his mad scientist fascination with the inhumans, they are beings new to him after he arrived in the future.
Now the last puzzle piece: the "holy sword". Anyone want to predict that it was the Ultimate Weapon made by humans during the invasion time to oppose the inhumans? It's just high technology that was cutting edge (literally, lol) just before humanity lost that war. Among its requirements to use it is likely "pure human DNA". So no human in the present time can use it as inhuman DNA got intermixed over centuries. Only someone from long before the war could be compatible.