If that were indeed true, then Bitcoin mining wouldn't be anywhere near as popular as it is now. Also note that my comments pertain to cryptocurrency in general (or at least the proof-of-work variety).

As for the useful computations you propose that ostensibly pay as well as cryptocurrency mining... which ones? If you're referring to something like cloud computing, keep in mind that the bandwidth requirements for cryptocurrency mining are basically negligible - which is an important factor to consider even in rural America, let alone rural Africa.