Bug reports.

In SoD, the central hex of the splash attack hits anyone, while the side hexes only hit living creatures (it's easy to test that this is indeed the case). It's the same in HotA. That's the whole logic. The concept of "friend or foe" is not considered in this mechanic.

well, okay, but are there any immune creatures?

There are no creatures immune to the central hex of the splash attack. Creatures immune to the outer hexes are the same as in SoD: any undead creatures: gargoyles, golems, elementals, undead.

and does it work the same way for Magog?

Yes, the principle is the same. Only creatures with immunity to fire (ifrit, fire elemental, phoenix) are immune to the outer hexes of the splash attack. In SoD, there are no creatures immune to the outer hexes of the Magog's attack.