This oneshot really lives and thrives on sexist assumptions, doesn't it ? Doesn't even have the courage to actually commit to saying them out loud, just "I know that you know". Her losing muscle mass and having a new center of gravity is a fine enough reason, granted, as is the gender politics around sports making things more complicated than they have to be. (i.e. men and women get weird when playing against each other in ways they don't against anonymous opponents)
But why would a practiced base runner lose a game of tag on her sex alone? I promise you that a female athlete will crush a male non-athlete almost 100% of the time. Your sex isn't some kind of modifier that determines your success in sports. It's a min/max thing that contributes -- and only contributes -- to your ultimate skill ceiling. And one that isn't nearly as influential in doing so as other factors like your base genetics.
Yeah I'm overthinking this. And I probably would've just rolled my eyes and moved on if they said the sexist part out loud. But I'm gonna call out gaslighting.