I believe many inland countries get their transit through neighbors. For example, I believe Rwanda gets ~99% of their data through Uganda/Kenya and Tanzania, and only a tiny bit goes through Satellite. I can't say for sure nothing goes out through the DRC or Uganda/South Sudan, but I'm pretty sure it's quite, quite small.

That is correct. Plus limited satellite uplinks. The undersea cable connectivity especially in East Aftica sucks. I have the feeling the primary reason why these cables exist is because they connect South Africa to the huge cables between Europe and East Asia.