That forgotten era before recorded history they mentioned in one of the earliest chapters when talking about the dragon of matter etc. , seems that it connected to human history of around 2000 A.D.
One panel of the moons in higher details seemed like it was showing Earth however, notably the islands of Japan. If that is the case I wonder if this world came to be populated with humans later and the merger of realms to some extent (similar to Witcher games and others) facilitated this migration.
Perhaps the dragons of matter noticed the willpower of the humans or human souls (& had them reincarnated into this world), since they said that humans had highest willpower due to their shorter lifespans.
The cyclopean vistas of massive giants walking in the distance and other faint threads of willpower coalescing look very similar and inspired from Evangelion and Nausicaa etc. Then there’s the whole Revenant angle utilizing a reincarnation variant concept which is also one of the key concepts from Evangelion, plus the guy getting hit by the truck and yelling “Ayanamiiii” (LOL) , as well as the giant mecha that was kept in the Quefto settlement, noticeably seeming alive to Sera and others when they first came across it. There is certainly a lot of references scattered around for keen eyes to pick up on.
Soo…..