With elevation (or high heat) at some point in ULH flights it doesn't matter how long the runway is - you run up against an aircraft's max take-off weight (MTOW) and/or the tire speed limit or brake inertia limits. You just need to make the plane lighter by leaving cargo, bags, passengers or fuel behind. This chart - straight from Boeing - shows how elevation degrades MTOW (and thus range). JNB has a long runway: 14,500 ft. On a 30 degree C day from JNB a 777-200LR will hit max tire speed before it runs out of runway, and can carry about 80,000 pounds less than it could taking off from sea level at the same temp. 80K pounds is 1/4 of a 77L's fuel capacity.

Delta's A350s - despite perhaps being close in range to 777-200LRs - may not be close enough.

source: http://www.boeing.com/assets/pdf/com...777_2lr3er.pdf