No train smash to get out of Arusha....
Ok with 1620mts early morning takeoff say 25deg max Flaps 20 minumum fuel (say around 9500lbs) should give you around 1450mts ground roll give or take 50mts - So I don't see a big problem here - * Remember the take off graphs are designed to include an engine loss at V1 and subsequent acceleration to Vr*
Ps The aircraft was leased to Ethiopian since 3 months to replace "Temporarily" the 787 while Boeing makes repairs.