Botswana's diamond mining industry is exactly one company: Debswana, a 50/50 joint venture between the government of Botswana and De Beers.
Which means, when you think about it, that half of one subsidiary of De Beers is effectively producing ~one third of Botswana's GDP. Really puts into perspective how exploitative De Beers is.
I would describe it at showing how poor/unproductive the rest of Botswana's economy is.
Or is the theory here that De Beers somehow keeps the part of the Botswanian economy they don't participate in poor/unproductive?
This is a difficult conclusion to draw given that the implied counterfactual is the assumption that Botswana could have had similar results sans De Beers. I don't think that's at all obvious. If my problem was that I had raw diamonds a few hundred meters underground and wanted to turn them into dollars, I'd probably call the preeminent experts on mining, refining, and marketing diamonds as well.
The venture began in 1969 and as difficult as the economic situation there is now, subsaharan africa was an even worse place then. So I think your counterfactual needs put into perspective. Additionally in 2013, the state created the 100% owned Okavango Diamond Company to export diamonds using the institutional knowledge they'd gathered over the decades.