Well it makes kind of sense, the flasks having a melee only mechanic and the malleable goo having a caster only mechanic.
That being said, the description on WoWHead just says "Apply Aura: Mod Hit %". Do e.g. the Anubisath Warder before C'thun also only reduce melee hit? The
debuff description is the same there (Apply Aura: Mod Hit %).