What's the point with that quite lackluster story in Endwalker?

[Divinemights]

Venat is not real and was written by modern people.

True, but it doesn't mean a character created can not have his/her own soul.
This is especially true with Japanese narration where a character usually have full background story, behave accordingly to the character setting and character setting fits to the civilization and ideology of the period.
In direct contrast, English Comics like Marvel is inferior , etc are weak in character setting and development and character often behave to the plot need instead of character setting
In her case, she is someone be from thousand of years ago and everything she has done fit her civilization and ideology