When I piloted aircraft, given they were only Tomahawks and Warriors, my elevators moved up on takeoff, pushing the tail down and the nose up, and away I, or we, went.
Every takeoff showed the elevators down, so what gives?
A different angle of thrust. Your jets were directed backwards only, and the only way to keep the nose up is to move the elevators up, just as in most planes.
These planes are focusing their thrust DOWNWARD, just behind the pilot. The NOSE is being propelled upward and needs no elevators to raise it. The ELEVATORS are primarily being used as FLAPS, during STO.
Did you notice that the exhaust was also vectored down? I wonder if this was to help with the ground effect or to assist the exhaust in vectoring to achieve more vertical push???
I wonder if the position of the elevators has something to do with the short take off. The lift fan up front and the downward pointed exhaust in the back probably makes a takeoff in an F-35 much different than a conventional jet.