You are wrong in that the concept of an elephant with siege on its back is NOT uniquely Khmer. There are several accounts of elephants with some type of stone or bolt throwing devices called ‘yantra’ on their back throughout India. However, where I do agree with you is that the Ballista elephant is by now iconic to the Khmer, so I would have preferred it to be kept associated with that civilization.
In fact, for my civilization concept for the Vijayanagara empire, I included a ‘yantra elephant’, but made it a mangonel replacement to differentiate it in that regard (even though it likely may not have been used entirely in that context).
But regardless of where it belongs, I really don’t like the current design of the Springald I mean Ballista Elephant, it looks way too slapped together. Who is operating those winches that currently extend 1m off the side of the elephants?