Radek could have replied to mjg59's post and said "There's socially-constructed gender, and there's biological sex. There are very few times when sex rather than gender is relevant: with your doctor, with your sexual partners, and--I would argue--in sports, for safety reasons. I have been discussing sex, not gender, although I have been a little sloppy with the terms."
That would be a coherent non-transphobic view, albeit one that is mostly adopted as a dog-whistle.