Yes nine women can make nine babies in nine months, but they're all going to be born in 9 months (give or take), which doesn't help you have a baby in 1 month. If you plan it right, you can have 1 baby a month with a 9 month delay, but again that's 8 months too late for a deadline.
The saying here is essentially akin to too many cooks spoil the broth. Put it this way: 1 brick layer can lay 9ft of brick in 9 days (no factuality here). If you're building a wall you can only go one row at a time. 2/3 brick layers will speed up the process. However 9 brick layers won't produce a 9ft tall brick wall in 9 days because 6 brick layers will be sat on their arses drinking coffee.
9 brick layers can produce 9 9ft walls in 9 days, but won't be producing a 9ft wall in a day 9 days continuously working as a team.
The statement doesn't preclude that work can be produced efficiently, for example you could easily have 4 brick layers build 4 9ft walls - AKA a house - in 9 days.
The aim is to find your fundamental limit and work with it. If you need a baby every month, sure plan ahead and you could have a baby monthly starting in 9 months, if you can account for that 9 month waiting period. You will never get a baby in 1 month with a team of 9 women and only 1 of them being pregnant.