The titans were weak against fel energy but what would happen to the old gods? Would they take damage, be immune to it, or absorb it and get stronger? It's like using kryptonite to fight Superman then Shazam. Superman might be weakened but Shazam would simply ignore it.
Also did the old gods get weaker during their imprisonment or stronger? They might have been absorbing energy from the Well of Eternity since the Sundering.