How do you figure? While his claim doesn't make any sense, he does say it:
"The world has about 6.8 billion people. That's headed up to about 9 billion. If we do a really great job on new vaccines, and health care, reproductive health care services, we could lower that by perhaps 10 or 15%."
The speech is on controlling carbon emissions and that unless we do something the will continue to grow.
The statement was made in the context of his overall argument is that population is going to grow by 9 billion and that there is not much we can do about that, at most we could lower the growth by 10=15%. So Gates concludes we need to change our energy sources to sources that emit less carbon.
If you were going to use vaccines to reduce the population, you could get a lot more than a 10-15% reduction in growth. Which led me to conclude before the Gates Foundation ever defended the comment, that Gates was being misunderstood and quoted out of context.