Indeed, Wikipedia and the CIA do not have data that corroborate that statement.
http://en.wikipedia.org/wiki/Demographics_of_Angola#Ethnic_g...
https://www.cia.gov/library/publications/the-world-factbook/...
The above 20% is probably wrong.
Just note that the citations from the CIA world factbook is probably worthless. In the official demographics of South Africa for example, neither illegal immigration or temporary migration is noted.
This is fairly high though (as much as 3 million Zimbabweans and many people form other African countries such as Somalia).
I read this in a South African newspaper. But in fairness, without a citation this is worthless.
I see posts claiming a 100,000 works in the formal sector (i.e. working for formal companies). The number of Chinese working in the informal sector or illegally would be unknown.
But the 20% figure is most probably wrong.
Do you have a citation of a source that says this? I just tried the usual kind of online sources for national population statistics for Angola, but I haven't found confirmation of this interesting statement.