The issue of South African farmers setting off to farm land in other African countries is not new. I have written before about the significant distinction between land-grabs and successful commercial farmers being offered the opportunity to establish successful commercial farms on previously (mostly) unproductive land. PLAAS disagrees. In the person of Ruth Hall, they argue that the land these South African farmers are being offered is not empty, that poor people are eking out a living of subsistence crops on abandoned former state farms and government owned acres. Even the construction of the term "the new great trek" is heavily loaded. The great trek, in South Africa, has strong associations of white settlers invading other people's land and killing the locals with abandon - a narrative which in itself should be problematized far more than it is.
It is possible, in fact probably, that there are groups who would like to lay claim to the land being offered (on long-term hire agreements) to the South African farmers. I'm not convinced that should be the question. The question is not whether this land is empty, the question is whether this land is productive. As much as food security has shifted focus from production to entitlements in the last few decades, countries with thriving commercial agriculture sectors tend to be better able to feed their populations (including poor people), both through production and through agriculture-driven economic growth. These countries have good farming land which is currently unproductive or under-utilized. South Africa has experienced, educated commercial farmers who have a proven track record of success. Under these deals, the land remains in the hands of the African government, rented out to the farmers who will, in exchange, create the kind of commercial agricultural success the farmers' own government will probably be wishing for in 10 years time.
The argument that there might be people who want this land, is not an argument against establishing effective commercial agriculture; it is an argument for putting pressure on governments to ensure that people are compensated (through jobs, services, etc.) for the land that is lost to commercial agriculture, through these deals and more generally. Sometimes subsistence farming is not so precious that it overrides the need of a continent to feed itself and the rest of the world.