I can only assume that since the ants get sucked into a vac that occasionally a shot of something terminal gets sucked in too. I don't believe the suction alone would kill those beasties, because as soon as the vac is shut off they're going to crawl back out the hose. Relocating a mad bag of fire ants wouldn't appeal to me.
When I lived in SC we had them. I was mowing the yard and all of a sudden started feeling the bites on my legs. The only time in my life I deliberately dropped my jeans in public on the front lawn! Poisoning the mound killed a host of soldiers, but the main colony just moved underground to a new location. Killing the ones sucked into the vac will be important as well as getting into the nest proper and getting the queen to totally wipe out the colony. If you don't get the entire nest, you may at least get them to move to where you could effectively poison them in the ground.