The "rules" come from a website wher some guy trying to be funny, collects perceived cycling etiquette.


However, the bias against sleevless comes from an actual rule. You can't race in an USA Cycling sanctioned race in a sleeveless jersey. Hence, Bike racers tend to find sleeveless jerseys (particularly on men) to be inappropriate.

Triathlon has no such restrictions. Hence sleeveless jerseys are more accepted by triathletes and their ilk.

Of course, it appears you can do a triathlon in a banana hammock, which does not suggest that either are actually appropriate.