Tldr: Disney committed lemming genocide
From White Wilderness, as the movie shows drowning Lemmings, put there by Disney employees, the narrator goes:
"And so is acted out, the legend of mass suicide and destruction of a species."
Probably because it's inaccurate/exaggerated in a couple ways.
1. Genocide refers to humans, not animals, and requires the act to be targeted at a group of people that share some sort of traits (typically race, or cultural identity).
2. Disney imported "no more than a few dozen lemmings," which doesn't really meet the numbers the word typically is used for.
https://www.snopes.com/fact-check/white-wilderness/
Disney killed lemmings, and unless they attempted to wipe out all lemmings that share a particular cultural identity, it's probably pretty insensitive to use it in this context.
One might also consider that animals are people, too.
However, I don't think you can make the case that Disney set out to wipe lemmings from the earthwe.
"the deliberate killing of a large group of people, especially those of a particular ethnic group or nation."
While xenocide is described as:
xenocide (plural xenocides)
The killing of a stranger or foreigner. (science fiction) The genocide of an entire alien species. (US, colloquial) The intentional killing of an entire foreign (plant or animal) species.
https://en.wiktionary.org/wiki/xenocide
Either is fine really, just seems like a semantic curiosity.