It is true. The Bantu farmers have been moving and displacing the hunter gather tribes that were the earlier inhabitants of much of sub equatorial Africa for over 1000 years. The Dorobo were actually not a community as such but bands of hunter gathers who were called Ogiek/Dorobo by their more advanced farming neighbours (Bantu- Maasai etc).
The Dorobo/pygmies etc got assimilated by Bantu farmers and were pushed further into forests, only few hunter gatherers remain such as the guys who live in Boni forest (Wa-saanya) who speak a language strange to Bantu ears.
As a matter of fact, most of Central province place names have no meaning in the Gikuyu as spoken today, meaning other 'tribes' named them.
For bed time reading:Guns, Steel and Germs, J Diamond, 1997. Read the chapter How Africa became Black on how Bantus colonised sub-saharan Africa.