Imagine watching these professional athletes who've put in time & effort training to be the best they can perform, and your conclusion on why they're so skilled is "the guy who gives them massages must be doing a really good job"
This might be the world's first anti-sports sports manga
First, you’ve got to remember that it’s Mirei saying this, and she’s naturally going to view the game through the lens of her profession. As someone whose job is to maintain athletes’ physical condition, it makes sense that she’d focus on stamina, recovery, and how well they’re holding up over time.
And more importantly, she’s not crediting Kouyou for their actual skill level. She’s not saying they’re good
because of him. What she’s pointing out is that his work allows them to push through fatigue and maintain performance deep into the match—basically, he’s enabling them to sustain the level they’ve already trained for.