According to Guns, Germs, and Steel, how did the Bantu come to dominate sub-Saharan Africa?

According to Diamond, the Bantu came to dominate
Sub-Saharan Africa in much the same way that other societies came to dominate their
areas.  The Bantu became dominant because they developed agriculture and technology
before any of the other cultures in the region.

The Bantu,
Diamond says, spread across Africa and supplanted people who were hunter gatherers. 
They continued to spread until they reached the east coast of Africa.  At the same time,
the Bantu became the first in sub-Saharan Africa to smelt metal.  This gave them a huge
advantage over other peoples in the area.

Diamond argues,
then, that the Bantu dominated their region because they got agriculture and technology
before other groups did.