Can you ask the SSS the reference for that?
If we're going to base it on the SSS Law, there is no statement that says the start of maternity is on the delivery date.
It all goes back to what was filed on the leave form.
If you filed a leave already on September 1 stating that you are going on a maternity leave, that is your start date.
If you still have Vacation Leave, and you filed a VL that leads up to your delivery date, then your Maternity Leave start on the delivery date.
Also, check what you have filled up in the SSS Maternity Reimbursement Form, Part I (to be filled up by the member). There are 2 dates that you need to fill up, 1st is start of maternity leave and the 2nd is the day of delivery / miscarriage. Question is... what was the date stated on the start of maternity leave? that will be the starting date.