It's relevant to the definition of genocide. The near-extinction of the North American bison wasn't genocide. Buffalo are not sapient beings.
That sort of dodging doesn't work in the Trek universe, where it's not really always possible to tell whether the victim is sapient even after fairly careful study. From the point of view of many of the Federation's enemies, eradication of Homo sapiens
would not count as genocide by such impracticably narrow terms.
And the only reason the bison slaughter wasn't genocide was that it failed - otherwise, a genus would have been killed. So the charge against Man would be attempted genocide there.
Okay, but then at what point does the Federation consider genocide an acceptable response to deal with an enemy?
That must vary from enemy to enemy. Some enemies can only
be defeated through genocide: e.g. the Borg are a single individual and defeating that individual means utter and complete destruction of the genus. Many other dire threats to the Federation have also come from a species consisting of a single individual, one that cannot be negotiated with, delayed, redirected or evaded - say, V'Ger.
This nicely serves to highlight what a meaningless term "genocide" really is. Sometimes it describes the killing of a single being or a small group in the accusatory sense, sometimes it describes the killing of trillions in the dropping of charges sense, because it puts irrational weight on the degree of completion of the act.
I am pretty sure killing a single being is not considered genocide in the same way as killing millions of people is.
I don't think Hitler would be as reviled, if he only killed one person.
For example, if someone was being mugged and that person killed that mugger in self defense and it turns out that mugger was actually the last member of a race, I don't think that is comparable to a government deciding to kill millions people, because they thought their country would be better without members of a certain race living there.