I don't know what actually happened, but throwing some numbers out there:
the runway at 7874 ft long. If the 737 stopped 2/3 of the way down, that would be 5250 ft of runway.
According to
this webpage the C-17 can land in 3000 ft with 160,000 lb of cargo aboard. That would be 57% of the available runway. US part 121 only requires that an airliner is able to land in 60 percent of the available runway (70% at a filed alternate). My current airline (and my previous airline) has an exemption for 80%, which we use frequently.
Add to that the fact that they almost certainly were not carrying 160,000 lb (72,500 kg for those of you still clinging that obsolete fashioned metric system) of cargo. Fuel load is anyone's guess, but it's worth noting that they went form there to Entebbee which is about 550 km away, so didn't necessarily have a lot of fuel on board.
FOr further consideration is the fact that about a year and a half ago a C-17 landed on a 3400 ft runway at Peter O knight airport in Tampa, Florida, thinking it was MacDill AFB
Soooo, it would certainly be *possible* to land a C-17 on the runway at Juba with a disabled 737 2/3 of the way down, and you'd still have a considerable safety margin.