They really should consider using terminology that the players understand, because "Group Boss" causes confusion every time. Logically players see a "Group Boss" not only as World Bosses, but public dungeon, dungeon and trial bosses, too.
@ZOS_Kevin is there any chance they will make this more clear in the future?
So yeah, apparently even in help pages and the like they use the term World Boss and Group Boss interchangeably.
They should just modify the endeavor to say complete a Group Boss (World Boss), and on the tip page explain that World Bosses (Also known as Group Boss) is needed to be killed.