This migration history is even more complex because the migration was in waves - Bantu who went to South Africa later migrated north again, meeting Bantu who were already there from earlier migration. You can tell this from the fact that some southern African languages have closer linguistic relationship to some East African ones, than they do to other Bantu languages in their region.
My dad (Bukusu of Western Kenya) once met a gentleman from the Venda tribe of South Africa, and they conversed in English for an hour until the South African got a phone call that he answered in Venda. My dad understood practically every word that was said on the phone, and they each discovered this tribe they'd never heard of before had more language overlap than their neighbouring Bantu tribes back home.
It's fascinating if you consider just how big an area we're talking about, and how much opportunity existed for other languages to wipe out Bantu dialects. I'm no linguist but I think Africa has a propensity for multilingualism that has kept these tribal dialects alive, and aside from DNA this gives the strongest evidence of the past migrations of Bantu people.
>My dad understood practically every word that was said on the phone
It is quite uncanny how South African languages sound like Swa. I always find it amusing.