All warden pets are immune to "aoe" hits. So are pets from cavy eggs, and a few vanity pets. To my knowledge, the priest fairies are not immune.
I agree it is bumkiss that certain "aoe" hits require a target to fire, ie potws, piercing arrow, ss, cleave.
As a wd/s in siege it seems my chiron gets none of the defense benefits I have as it always gets hit for twice the amount of damage as me, both critical and normal hits, according to the combat log. So realistically just use single target hits on the warden pets, because a "class balance" to adjust their immunity would greatly affect pve far more than both wardens or players running with wardens would appreciate.