Let’s worry about letting people be their authentic selves first, which may well involve dropping gendered sporting boundaries (which primarily exist to protect male economic and social domains from female competition, anyway).
It is laughable and dangerous. Stop.
This isn't true. Sexual differentiation begins around the sixth week of embryonic development. Prior to that, the gonads are undifferentiated.
> there's no reason to believe the fastest human would necessarily remain biologically male, if the dietary and training regimes were entirely equal
But we already have evidence that this is the case, in world records for elite athletes, for whom training is optimized: the male times are significantly faster than the female times.
> gendered sporting boundaries (which primarily exist to protect male economic and social domains from female competition, anyway)
The rationale for women's sport is that it is fair to exclude male physical advantage from competition. The primary reason for existence of the female category is fairness. In contact sports, there is also safety to consider.
If competitions were mixed-sex, they would be dominated by male athletes in almost every sport. Again there is plenty of real-world evidence for this.
Look there is nothing wrong with allowing people to be their authentic selves, just don't create the worlds fastest women runner be a man.