No knowledge of game code here, but lots of trial and error testing. From my article Understanding Pet Damage:
Also, any pet attack that is either a magic spell, AoE, and/or anything that isn’t their default basic attack (including any modifier-granted attacks) generally won’t have % weapon damage, and thus won’t benefit from flat damage bonuses. For the xpac, this means pets such as skeletal mages or any of the blight fiend’s attacks (other than his default “Blighted Claws” melee attack) won’t benefit from flat damage.