Earlier quoted context omitted.
The general opinion is this is a synthetic diamond laundering operation. Synthetic diamonds are of course chemically identical to natural, but natural diamonds still fetch a higher price.
Do you have a source for that statement? If I'm not mistaken synthetic diamonds are almost always detectable by experienced gemologists (like the ones working at Debeers, presumably), and the timeline of synthetic diamond technology wouldn't line up either.
You are right though, gem-quality synthetic diamond technology didn't come around for several decades after this mine started producing.