I think a bizarro definition of "genocide" is being used here.
Well, it's a bizarro concept. "Killing of groups" is meaningless. If I kill two people, it's certainly a group, but is it genocide? Only if it turns out those two were the entire population of Big Purple Rock Island, or the only two followers of the Lesser Pumpkin Fairy. If I kill sixty million soldiers, is that genocide? Only if they come from an all-warrior culture and it turns out I thus killed a significant percentage of that culture.
As used since its introduction, "genocide" is a thoroughly racist term, establishing that killing is bad if it touches a "preferred" group of people. Like libel, it's one of those crimes where the defendant cannot know for certain he is committing it until the plaintiff establishes that he indeed "experienced" the offense in that particular manner...
Starfleet doesn't believe in orbital bombardment of cities.
As per GO24, it does, at least to the same degree USAF believes in the nuking of Russia. But the UFP doesn't appear to believe it needs to have Starfleet exercise that option, as far as we know.
What we know is very little, though, as we only witnessed very small and peripheral elements of the Dominion War for the most time. The one big fight involving a planet (or a pair of those, apparently) was that of Chin'toka, but we can't really tell one way or another what was going on wrt orbital bombardment. And how does beaming down Klingon troops differ from orbital bombardment in the genocide sense?