We know about Mali, because there are copious written records about that Kingdom. So for obvious reasons we know less about Kongo due to the lack of written records, but a while back I did some on Kongo and I remember reading that a researcher believed that Kongo's trade contacts may have extended as far inland as as the Swahili City States. The thought stemmed from the fact that some artifacts were found in those city states that were only produced in the Kongo.
It is good stuff either way.
Do you have the source for this? Because this is VERY INTERESTING! And I am a BIG fan of the Swahili coast. Maybe if you have time you can inform us more on the Kongo Kingdom.
Please do not take my post as belittling the Kongo Kingdom. All I'm trying to say is that the Mali Kingdom had a more international reach compared to the KK and dominated the most precious resources at the time(gold/salt). But if you have time PLEASE post in the Kongo Kingdom tread I linked.