Perhaps Stratford Bill meant that comparisons can be most offensive if the reference is to objective events or real persons, clearly because of the very complexity of life itself. But to use contrasts as a sounding board to describe how conversely people may act when they are attempting something stressful, is not such a bad exercise. Especially if we can point to certain traits in various sporting events that those champions must possess.
People come in all shapes and sizes.
Some are drawn to a sport because Mum or Dad find it convenient or big brother and sister play it or for any one of a hundred reasons. Almost none are officially identified as talented in a certain direction and pointed towards the sport that suits their structure or inclination. These days in Australia there are certain States who are doing just that - identifying dexterous young people and the sports best suited to that particular proclivity. There are few things more painful than to see young persons deeply involved in a sport which obviously does not suit their structure when they could be enjoying more success in a sport that does befit them. In swimming, and in other similar types of competitive sports, contestants require essential attributes to take them to the top and it is not intended to bore the reader with details of strength and endurance and all the other factors ad nauseam.