We know that's not true. The Romans had no problem repelling them at the Battle of Zama. The legion or phalanx would just need to be trained to make noise to frighten them and open up lanes for the elephant to charge through (they weren't keen on charging into a wall of spears), and then you have a dedicated group of skirmishers to hunt the elephants down once they pass the line.
But I think the idea here is that the elephants in question are at a similar level to humans intellectually. They will be able to overcome fears (be courageous), plan with others, use tools, etc. Hard to know how would come out on top in that case. Probably would come down to food consumption, reproduction rates, adaptability to various environments etc. more than who would win in head-to-head battles.
The lack of opposable thumbs would significantly limit their ability to craft tools. Even if they could use trunks to craft melee or throwing weapons, there is no way for even the most intelligent, coordinated elephants to overcome a ditch with a line of ballistae or cannon on the other side. Cavalry were able to deal with this due to their relatively thin profile and high maneuverability, and even then ditch + spear wall + artillery or ditch + grape shot is a pretty effective counter.
It would be interesting to see them execute the Fabian strategy though.
