In my opinion, thousand years ago, when the moon civilization descendant down to earth, they brought their literacy with them to the blue planet. As they need resources what better place to ask than to the powerful Ancient Kingdom? I would also like to bring your attention to individuals like Pudding- who is a mix blood. This technically means, Big Mom had physical relation with a third eye tribe dude for her to be able to have Pudding. The reason I bring this up, is very crucial for this theory. This would indicate the possibility of mix race existed at that point of time in the past as well. Knowing/Assuming the Ancient Kingdom’s ideals of “everyone under the same roof” would it be far fetched to assume, the Kingdom had people from all kind of races?- probably not.
Thus, it would also be highly likely that seeing the abundant resources of the blue planet- the three who descendant from the moon chose to stay down. Now, here I would like to elaborate, even if in the Enel’s cover story we see depiction of three individual descending down to blue planet it could very likely mean that the three depiction of individuals represented 100s of their civilization. I mean how can there be a civilization of just three people right? Thus, people of moon shared their technology of advance craftsmanship and weaponry, while the Ancient Kingdom provided them with basic needs and resources they needed. This would explain, how one single nation living among other nations rised up to the top in terms of strength and advance way of life style. In real world we hear fiction about aliens, and in One Piece- the knowledge from moon civilization. This however would not apply for Poseidon. Poseidon clearly is a living creature in itself- so there’s not point of “creating it”, thus the knowledge would’ve been there to only build Pluton and Uranus. As they chose to settle down in the Ancient Kingdom, relationships started to form. I will even push my luck to an extent by saying, the king of Ancient Kingdom married one of the three (either Skypiean, Birkan or Shandorian) as his wife. The existence of ancient text would not make sense if this was some kind of language spoken by Kozuki Family or the Ancient Kingdom. However, it would make more sense if those literacy came from a tribe distinct of the humans or any other races living down below. The knowledge of ancient text may have been passed to Kozuki clan by one individual: Kozuki Toki. Toki in my speculation was a child that grew during the rule of Joy Boy as the King of Ancient Kingdom. She grew older learning the history of the Ancient Kingdom and moon civilization, reading and understanding the ways to write and learn the unique ancient text and also about the existence of Ancient Weapons. However, as she just approached her youth the war was approaching near. But her fate was not be decided during the Void Century. During her early childhood, she had eaten time-time fruit. As the wrath of war begun, she used the ability to travel forward in time. From this point forward, theory becomes self-explanatory. Joy Boy knew about Toki’s ability. He guided her to inscribe those text to keep their history safe and also to fulfill his promise. Completely situational, as she travelled forward in time, she ended up meeting the descendant of the Ancient Kingdom- a clan known by the name Kozuki. And ever since, she jumped forward in time, to pass down the knowledge of ancient text, the skill to inscribe the text and also how to read it. And then, Kaido happened.
