How did the Boeing 777X's main landing gear lift before the nose on its first takeoff?
Correct on both guesses to some extent I would say.  The airplane is pretty light, and with flaps down the flapped area of the wing is operating at a positive AOA (remember that the chord line is from the LE of the slat to the TE of the flap, so with flap down there is positive AOA even while 3 point). 

Also as it's an initial test flight I wouldn't be surprised if the pilot delayed rotation past normal Vr 5 or 10 kts just to have some extra energy margin if something went amiss, so not necessarily holding the nose down, but delaying pulling it up.

Added together, maybe helped by a bit of crosswind coming from that side, you get the truck starting to come off before rotation.