I use the term genocide to describe the Sundering not only because Venat's actions fit the definition perfectly but also on the basis that there's characters in the setting who use the same phrase to describe the actions of the Garleans on a narrower scale - certainly more limited than anything Venat or the Ascians did. Furthermore various protagonists are pretty stark about the harm the Ascians caused.

At the end of her day, it was her explicit goal to end the Ancients as a species to bring about the Sundered because she believed they were better equipped to cope with despair. I will leave aside how little in the story even supports that, but it describes her goal. In wanting the Sundered not to be "beholden" to the past, she also aimed to wipe away all memory of the Ancients. There are few better suited terms here than genocide.

It doesn't suddenly cease to be an act of genocide simply because the story desperately tries to portray the individual responsible as a self proclaimed 'supreme deity' or a 'herois'.

Personally speaking, as someone who has discussed the story actively since the days of ARR I never saw any reluctance to use the term genocide until it was Venat who happened to be the target. I think there's a lot that could be said about that, though everybody is free to use the forum's - admittedly dated - search function if they're curious to see evidence of the term being used freely in lore debates prior to Endwalker and Shadowbringers.

The French localisation also describes Athena's actions as genocide, as an aside - and she's a character that mirrors Venat rather closely.