Genocide actually can come from within a nation too. And we actually do have a description of the Sundering (written by Ishikawa) which is this one: https://i.imgur.com/CWFRPdy.png
Factually speaking, the Sundering resulted in the total destruction of the Amaurotine culture.

Why you want to call those other things "genocide" yet refuse to apply the term to an equally horrific and destructive event is beyond me. And we do have real world analogies for an event which forces a people to forget their culture and language: the thing I was subjected to, being taken away by force from my family as a child and subjected to forced cultural assimilation and religious conversion, my cultural identity was completely and irreversibly fractured by that event.

And also genocide does not require murder to be a genocide
"It is intended rather to signify a coordinated plan of different actions aiming at the destruction of essential foundations of the life of national groups, with the aim of annihilating the groups themselves. The objectives of such a plan would be disintegration of the political and social institutions, of culture, language, national feelings, religion, and the economic existence of national groups, and the destruction of the personal security, liberty, health, dignity, and even the lives of the individuals belonging to such groups. Genocide is directed against the national group as an entity, and the actions involved are directed against individuals, not in their individual capacity, but as members of the national group." - Raphael Lemkin, "Axis Rule in Occupied Europe"

The Sundering undeniably destroyed them as a national group, and Venat knew this would be the result and intended it because, as said by Yoshida in an interview, she thought her people's culture was leading them down a path of stagnation, to the same fate as the Nibirun. The intent was clearly to purge the culture in question from existence since it was the culture she had a problem with.