This card gives 15% to elephant attack/damage, but strangely not to War Elephants. The Indians have a nice infinite 8 War Elephant card in age 4, which is nice to add to your Mahout contingent for extra (albiet, weaker) meatshields. Obviously on a map without native War Elephants you can't upgrade them, so you have to rely on cards to boost their stats. The Indians get a native stat boost card, and their other melee damage boosting cards also seem to apply to them. But bizarrely the Elephant Combat card doesn't, and they are, after all, elephants.
I seemed to solve the issue by adding the line <UnitType>AbstractElephant</UnitType> to the unit in the protoy.xml file, but then this bizarrely made them cost 1 instead of 0 pop each (I'm not sure if the other elephant cards, such as the population one, started applying and messed things up). So I'm not sure how to get it to apply and keep the pop at 0.