[6.0 - 6.4 Spoilers] An interesting video detailing Venat's polarizing reception

[Absimiliard]

The first of the Sundered might’ve been reduced but they also had enough memory and coherence to make cave paintings of Amaurot as well as the actual Sundering with representations of Zodiark and Hydaelyn. I don’t think they were formless ghosts or drones for generations until people slowly reconstituted themselves like the End of Evangelion.

FFXIV's own writers wrote that stuff for the crossover event. We can't attribute it to Shadowbringers shenanigans either, as the Re[in]carnation side of their deal was implemented well into Endwalker's lifespan. For my part, I think they should've either left that bit out entirely or put it in the main game. Forcing people play the NieR gacha to get lore for this game was kinda scummy.

As for the cave paintings; one wouldn't need to have their previous identities intact to recall those things. It may not even have been the first generation of sundered who made the paintings in the first place, merely people whose souls yet retained some deeply ingrained residual memories of those events and the major players therein. Even if it were, I'm afraid scrawling some half-remembered things on a cave wall isn't indicative of being remotely intact. Even chimpanzees have been known to produce primitive illustrations of things they found particularly horrifying or interesting. Not to say the sundered were as chimps, mind; merely that the ability to draw things is not limited to those of high intelligence or even complete sapience.