There's some sci-fi (that I cannot remember off the top of my head) that actually uses this as a plot point.
I may be misremembering, but the idea is there's a special race of humans who have continued to evolve living in low gravity (space) environment, and the terrestrial human (those living on planets) governments would hire them as space mercenaries because they're at a huge advantage when it comes to 3d combat compared to training some terrestrial guy.
I had always thought of that line as metaphorical (“two dimensional thinking”), but that’s a very interesting idea, and fits well with the final battle that immediately follows it.
I may be misremembering, but the idea is there's a special race of humans who have continued to evolve living in low gravity (space) environment, and the terrestrial human (those living on planets) governments would hire them as space mercenaries because they're at a huge advantage when it comes to 3d combat compared to training some terrestrial guy.