That metric correlates far more strongly to fan interest (and sponsors’ willingness to spend money) than it does to how a female tennis star would compete against a male tennis star. Tennis is probably the one sport (at least, in the U.S.) where the women’s “league” is roughly as well-established and successful as the men’s league, and where fan interest is anywhere near as close.
Golf is probably the only other example which is even in the same ballpark, though I don’t think fan in women’s golf is nearly as strong as for men’s golf yet.
The equal prize money at grand slams is not done for any direct commercial reason. Men’s tennis gets more viewers and advertising. This can be seen clearly reflected in the prize money at men-only events versus women-only events; the men get paid significantly more.
The top women are not close to competing against the men. My son can compete with women in the 300s in the world, yet he is not ranked that high in just 18U in the US.
I believe someone could devise a test for the “bigger pool” hypothesis. A statistical analysis where one estimated the size of the male and female pool could be matched with the probability that a female would reach the top x amount with a given frequency.
If the women did not reach that level with predicted frequency (or in reverse exceeded the predicted frequency) the original hypothesis would be shown as incorrect.