Typically BA B747-400's take off from JNB at about 354 tonnes (ZFW about 232t + about 122tonnes fuel) for the 10:30 flight to LHR. This at about 15-20 deg C OAT and calm wind will require about 1.68-1.70 EPR from the engines, which is not short of full thrust.
Max EPR with packs off at that density altitude is about 1.74, ie the take off is pretty limiting, and frequently the EGT's will be about 740deg C (vs 785 max).
In those conditions, there is NO WAY that a Flap 20 take off will result in a successful conclusion with 2 engines not delivering thrust (with or without leading edge devices as expected).
There is a lot of bullsh1t here. As I understand it, there were spurious reverser EICAS messages, at the same time there were leading edge flap anomalies; but the engines still produced commanded thrust. What little excess thrust remained was applied, the gear retracted, and after at about 160'agl the leading edge devices re-deployed to the correct position.
Prior to that, stick shake activations had occurred at about 12'agl.