Because Nova Flame debuffs are not Incinerate debuffs Sorry to be *that guy* but Nova Flames are no debuff, which is why Havok is not immune. It's a passive effect that deals energy damage. The effect is all the same- that it isn't an incinerate debuff, but nailing down the terminology in this game can help avoid confusion such as in the case of the OP here.
Because Nova Flame debuffs are not Incinerate debuffs