I know this was a few pages back, but I believe the idea is supposed to be that the ranger is adaptable enough to apply the techniques they honed in fighting their favored enemy to novel situations. So, with hoardbreaker for example, it’s not that their favored enemy is “any two creatures standing next to each other,” it’s that through fighting a favored enemy that usually fights in large groups (let’s say it’s kobolds, for example), they got really good at fighting enemies that stand in close formation, which turns out to be useful not just against kobolds, but against any two creatures standing next to each other. The colossus slayer’s favored enemy may have been giants, but in their experience fighting giants, they learned to more effectively harry already wounded enemies.