I'm not sure why you think that because someone said that their EC was caused by trauma that therefore everybody that causes trauma to their lips will get that condition. Someone might need a (maybe genetic) predisposition for EC (or certain skin conditions in general) for it to actually "break out" after repeated trauma or other triggers.
The number if boxers and more importantly the number of boxers with that predisposition and even more importantly the number of boxers with that predisposition who "get the right amount / type of repeated trauma" might just be so small that you don't know any boxer who has this.. :)