Spoke with a friend of mine who is a retired triple seven pilot. Asked him about this and he told me it's not so much DA being a lmiting factor but more about what's called accelerated stop distance. a number that is calculated and determined by the aircraft manufacturer.
Accelerated stop distance sounds like it ultimately relates to how much runway is available. I understand about density altitude, but elevation is still the dominate factor and temp and humidity only modify the elevation from standard conditions. Plenty of runway at JNB, regardless of the DA.
BTW, I flew the Houston Express for almost 6 years, working 28/28 day rotation in Angola. We'd leave IAH at noon time for the 13-14 hr flight to Luanda and usually take about 8000 ft of the 12,000 ft available at IAH. One time the long runways weren't available, so we couldn't leave with a full fuel load. So we flew to Atlanta, home base to our charter carrier, the now defunct World Airways and fueled up there and continued on.