What I'm reading is that this is a computer generated image of a studio type recording of artificial thunder.
The "artificially triggered" refers to the lightning. University of Florida has a lightning research facility where they can induce strikes with rockets that have a trailing wire. I have seen photos and videos of this triggered lightning. They probably did this so they didn't have to wait for a "natural" strike.
There was a lot of post-processing of the sound recordings, but they captured the actual thunder with the microphone array.
Greg H.