I think that is exactly it. This response is solely from a TECHNICAL support issues (i.e. how the game performs, not rules and regs and ToS stuff). It's them basically saying "the game wasn't designed to play multiple instances on one machine, so we aren't going to help you if you try and something goes wrong".

Kinda like calling up a ladder company saying that you are having problems balancing 2 ladders when you stack one on top of the other to make it taller. Then the ladder company says the ladders weren't designed to be stacked on top of one another, so they aren't going to help you.