No, no, this isn't why.

Fractals are ubiquitous in nature. That people in ancient Africa either noticed these patterns and created variations on the theme, or re-invented those type of patterns separately, is cool, and quasi-mathematical, but it's not what Mandelbrot did.

And Africans weren't the only ones. You can find fractal-like patterns in mandalas (India), calendars (Aztec), art (Rome), mathematical patterns (Appollonian gasket, Greece) etc.. So cool, yes, up there with the best, yes, probably, but what Mandelbrot did...no.

You see, mathematicians had generated self-similar patterns for plenty long before Mandelbrot came around. For instance, the Sierpinski Triangle was described in 1915 by the Polish mathematican Sierpinski (but it wasn't a new construction then--you find the same pattern in medieval art). So why isn't Sierpinski the "father of fractals"?

What Mandelbrot did was systematize the understanding of fractals. Rather than merely showing off a handful of disjoint examples and some vague intuitions, he came up with specific properties and metrics to enable us to understand fractals (e.g. fractal dimension--he realized that there could even be such a thing as a "fractal dimension") and thereby turned intuitions about self-similar patterns into a precise field that could be explored in depth.

(Edit: no, I’m wrong…fractal dimension was actually invented by Hausdorff — though Mandelbrot had a wide variety of other contributions, and gave fractal dimensions their name. In so doing, he stole the “Hausdorff dimension” name from Hausdorff. Oh well.)

For instance, the Sierpinski triangle has fractal dimension of log(3)/log(2), or about 1.585, a precise measure of our intuition that there's just a lot more of it, in some sense, than there is of a straight line, even if it doesn't actually fully fill in a 2D plane. Previously, with measure theory, all you could say was that it was "of measure zero", meaning that if you don't count the area of all the holes you've cut in it, its total area is...zero.

True, and yet unsatisfying. Mandelbrot showed us how to characterize these sorts of patterns in a much richer fashion and pointed out how widespread they were — not mere isolated bizarre mathematical monsters or quirky patterns any longer.

So the reason Mandelbrot, and not Sierpinski, and not ancient peoples (African or otherwise) are credited with the "discovery" of fractals is that he gave us a mathematical framework with which to explore them and compare their properties.

Giving ancient Africans credit for this is kind of like giving Australian aboriginals credit for the field of aeronautics (or the Bernoulli effect) because some of their boomerang designs used an airfoil shape. It's good to know about prior art and it shouldn't be diminished (and too often we proclaim "this is totally new!" when in fact "this is a borrowed or rediscovered old thing, now greatly elaborated!"), but then again, it's really really not the same thing. Boomerangs are awesome. But they're not airplanes.

Credit where credit is due, but in this case there is different credit available for different kinds of contributions.

(Also, the original African adrinkas are related in name only to the physicsy ones, according to Gates, from your link: "the mathematical adinkras we study are really only linked to those African symbols by name".)