This is similar to the Russian manner of teaching the powerlifts to beginners - starting with partial movements (box squats, deadlifts off blocks, presses to foam blocks) and gradually increasing the ROM as strength, skill and flexibility improve.
FWIW, I don't actually think his back was neutral at all - it was actually hyper-extended, which is probably fine as a position in the olympic lifts, but not ideal for squatting efficiently and just as dangerous as rounding. This is a direct result of exaggerating the chest up cue.