As pp's have already mentioned Ross Tucker has already given one good explanation why the Phelps argument doesn't stack up.
Here's another.
There is perhaps the misconception that Phelps success is ALL about his physical characteristics - specifically he's tall but with a long torso and relatively short limbs, with large hands and feet.
It's true those characteristics make him ideally suited to swimming.
However he's not a unique phenomenon. In a world of 7.8 billion people of which roughly half are male, statistically speaking there will be not just one other but many men who share virtually identical body shapes to Phelps.
The reasons why they are not Olympic swimmers are vast.
Maybe they don't like swimming, maybe they don't have access to a pool, maybe they were never taught to swim as a child....etc etc
You see what made Phelps great was not just his physical characteristics but years and years (from childhood and before those traits were fully known) of coaching and training. Phelps himself also was willing to put in the hours and dedicate himself to the sport - something not everyone even if they had the latent talents could or would want to do.
Still, theoretically speaking there are absolutely men in the world who had the capacity to beat Phelps if they had access to training and the appropriate self discipline.
In the case of CS there are no women on the planet who have the same traits - because CS is genetically male, thus no woman who could with the right training/self discipline "match" their performance.
CS isn't an outlier in woman's sport - that rare combination of physical characteristics/training/dedication like Phelps is in men's swimming.
CS excels not as an outlier but as a intruder into a class they should not be able to compete in because the advantage of male puberty and testosterone provide a level of advantage no woman could match ever.