My interpreation is that the barbarians make the most damage to the primary victim but nothing is said of most damage to all players so it is up to the controlling player.
Other rules for other calamities require the most damage to secondary victims.
But as written, the rules for Barbarian Hordes seem to allow the controlling player to choose between more or less damage to other nation's units in the way as other nations are not considered "secondary victims" that must suffer maximum damage.
That was the intent. As long as the primary rule is followed the secondary fallout is not considered in the rules.
Jennifer Schlickbernd Advanced Civ design team.