Well the short answer from the quoted paper where that measurement came from
"The channel is more like a high-gradient mountain stream with a very
large discharge; which is consistent with the theory that a short mountain river eroded
through the divide and reached Malebo Pool, forming the present Lower Congo
This is the location where that measurement came from.
However reading the rest of it, this is an anomalous depth and the majority of the river co forms to more normal standards and is around 10 to 80m on average.
Both [the height profile](https://i.imgur.com/GJWZEKs.png
) and the sheer volume of water (42,000 cubic metres per second) give the river an insane amount of erosion power, carving a far deeper channel than, for example, the Amazon which has an [extremely shallow profile](https://media1.britannica.com/eb-media/55/103655-004-81742AE5.gif
) over its lower course. (EDIT: In fact the amazon takes around 5000km to drop from 200 meters altitude to sea-level. The Congo does the same in just its final 100km.)
The first European to explore [the Livingstone falls](https://en.wikipedia.org/wiki/Livingstone_Falls
) (the long, steep stretch above Matadi), Henry Morton Stanley, said of them:
*"..the wildest stretch of river that I have ever seen. Take a strip of sea blown over by a hurricane, four miles in length and half a mile in breadth, and a pretty accurate conception of its leaping waves may be obtained. Some of the troughs were 100 yards in length, and from one to the other the mad river plunged. There was first a rush down into the bottom of an immense trough, and then, by its sheer force, the enormous volume would lift itself upward steeply until, gathering itself into a ridge, it suddenly hurled itself 20 or 30 feet straight upward, before rolling down into another trough. If I looked up or down along this angry scene, every interval of 50 or 100 yards of it was marked by wave-towers - their collapse into foam and spray, the mad clash of watery hills, bounding mounds and heaving billows, while the base of either bank, consisting of a long line of piled boulders of massive size, was buried in the tempestuous surf. The roar was tremendous and deafening. I can only compare it to the thunder of an express train through a rock tunnel."*