Short legs have nothing to do with fitness, but they are an advantage for competitive swimmers, if combined with big, wide feet, long arms, and a long torso. An athlete's legs tend to be muscle and bone, which sink.
I was very interested in swimming (non-competitive) as a teen and young adult, but I have a short torso, long legs, and narrow feet that are small feet for my height. My long arms were my only advantage in swimming. I am sure it would have been funny seeing me trying to swim freestyle, with my feet and legs sinking as soon as I got tired. Being non-competitive, I quickly learned to favor other strokes that let my long legs work more efficiently, such as the trudgeon and sidestroke.
Long legs favor distance runners, who are no less fit than swimmers, just suited for a different sport.
__________________
Have Colt, will travel...