I would agree. I think that all Zao bosses also have 1h despawn but the confusion stems from pretty much all of them spawning together with creatures but not always themselves, if we look at how the old leaked files defined raids:
# Ort: Im Ritualraum unter der dunklen Elfenstadt Shadowthorn
# Monster: Dharalion (besonderer Elf Arcanist)
Delay = 0
Position = [33034,32175,9]
Spread = 1
Race = 203
Count = (1,1)
Lifetime = 28800
# Ort: Im Ritualraum unter der dunklen Elfenstadt Shadowthorn
# Monster: 2 Elf Arcanists
Delay = 0
Position = [33034,32175,9]
Spread = 3
Race = 63
Count = (2,2)
Lifetime = 28800
# Ort: Im Ritualraum unter der dunklen Elfenstadt Shadowthorn
# Monster: 2-4 Elf Scouts
Delay = 0
Position = [33034,32175,9]
Spread = 3
Race = 64
Count = (2,4)
Lifetime = 28800
In the example above there always spawns 1 dharalion, 2 elf arcanists and 2-4 elf scounts
The most likely assumption we can make is that, to use Battlemaster Zunzu as example, has a row of:
Count = (0,1)
meaning either 0 or 1 can spawn. That Zaoguns have a count:
Count = (0,2) or (maybe 1,2 even as I have never seen one without Zaoguns)
And Fleabringer has 0,1, with insect swarms 2,4 etc.
Probably this has to do with them all being multiple spawns and there was an idea about adding some "more" randomness to the boss spawns. Im pretty sure Hatebreeder does not share this trait of 0,1, and it only has one spawn location.
