Did they really need Facebook's capital? The market was absolutely flooded with VC money, Magic Leap raised almost $3B since then.
Facebook might have helped but from a purely technical perspective, Facebook wasn't the only path.
Currently the Quest is the only Android based (and thus fully stand alone) headset that's wasn't Daydream branded and wasn't killed when Google mothballed their VR support. There's less now then there were, or would have been if Google stuck with it.