>Why the downvotes?

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.

Genocide is by definition against a group having common genetic/ethnic identity.

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.