Don't think you quite understand why and how population growth occurs. Have you noticed how population growth is highest in underdeveloped and developing countries and drops as a country becomes developed? If you give people access to good medical care and vaccinate them against diseases there is less likelihood of dying either during childbirth, early childhood and late childhood-early adulthood so you don't need to have as many kids to allow for that risk.
So Gates is perfectly correct to say what he did. If you also listened to the whole talk you would have heard that he felt that population was one area where it would be very difficult to make the significant difference required and the real key areas were energy efficiency, CO2 storage and zero CO2 energy supplies.
Its actually a very good talk but you have cut 2 minutes out of it to try and make it about Gates trying to lower global population using a COVID vaccine.