Census data, just like most data as far as foreigners are concerned is very inaccurate, firstly, illegal foreigners will make sure that they are not part of anything official in the country, why would you jump the border illegally, avoid being documented and then willingly provide your information in a government census?
They will duck and dive, if they are cornered, they will provide false information, census takes your word for it after all, so if a Malawian tells you they are Zulu from kwaNongoma, then you record that.
Lastly, Census 2022 was deemed very ureliable, I was never physically counted, for the first time in my life since I knew census was a thing, no one in my complex was counted, I ended up doing the voluntary self enumaration thing because I am a good South African.
Honestly people are seeing through all the BS of trying to downplay the problem of illegal immigration, it's just a waste of time, illegal immigration must be addressed, it causes a lot of problems, don't try and tell people they are imagining things, and no, no one said it will solve all our problems, whatever problems we have after that are our problems and we will deal with them without illegal immigrants.
There are serious concerns about the reliability of the census data
groundup.org.za