This is the trouble with entering that margin between what SOPs normally allow and the actual edge of the flight envelope. Trying to get airborne without sufficient thrust at low speed will put you there. Another aspect that seems to have been overlooked here is the effect of raising the gear in such a compromised performance situation. Back in 1974, Lufthansa crashed a 747 on takeoff from Nairobi. The leading edge devices were not properly extended and the aircraft staggered into the air just shy of the stall. What little lift they had was lost when the gear was selected and the doors opened up to expose the large void in the fuselage which accommodates the gear. The aircraft ploughed into the ground and the aircraft was destroyed.