I think they're trying to say that if there was a mixed league it would always end up being 100% men at the highest level. Like, mixed league basketball would almost certainly be just men at the highest level because of how the sport works.
So maybe I think what they mean by "mixed league" is not a "Maybe Mixed League" but like "Definitely Mixed League" as in mixed participants being a strict requirement somehow?
Mixed league works fine for your company softball team. Not so much for professional sports.