If you have a look to the East of MB6 & MB8 you will note a...

If you have a look to the East of MB6 & MB8 you will note a change in elevation of around 100m over 4km distance.
You do not need a river to transport the diamonds over such a short distance and a 100m drop in elevation.
See the link below on how different upheavals shaped the area East to West, literally shuffling the gravels without the need a river systems to transport the diamond bearing gravels over many millions of years.
My hypothesis is that just because the concession border is 5km away, it does not indicate that the diamonds may come from outside the concession border.
https://link.springer.com/chapter/10.1007/978-3-031-18923-4_4