"This" was referring to what happened in the article. A bunch of parents worried that a seemingly cis girl was secretly trans and had a competitive advantage. As I stated above, the biological differences in sex hormones that allow people who have a primary sex hormone of testosterone to pack on more muscle to their frame, grow larger lungs, etc. are pretty noticeable when it comes to an age where this matters. However, they don't reveal what sport this was, and in most sports the sex divide in performance has been almost completely erased when you compare by weight class instead, making this investigation even more questionable in the first place.