He manually deciphered ancient scripts on his own. The only thing night owl told him was about the inner working of the government and the invasion. So eldians bringing prosperity is the most reliable history we know so far.
For example, romans brought a lot of technological advancements, but what did the germanic tribes do? They destroyed them and sent everyone back to the stone age. They then started spouting shit about master races when they're really just jews themselves. It's basically what's happening here.