In some video games you may randomly face an AI opponent without the game being explicit about it. That way the average human win rate can be above 50%. I don't know yet if this is good or bad.

Nor any actual group of people. This "optimal" is zero sum: someone else has to have a lower winrate in order for it to work.

And yet when you see a professional boxing match, both boxers' win rate is usually above 90%, not 50%.

A boxing career has approximately 0 rated fights. A cheap career has hundreds to thousands.