You're making the assumption that Bungie is thinking of the word "Acausal" that's in line with what its real world definition is. I don't believe that they are. The Vex are capable of fanatic things, but always things that are explained within the laws of physics and bound to a chain of connected events. Paracausal means that there is no chain. The Paracausal being ignores chains. They demand x and if their will is strong enough x just is. This is how the Traveler terraforms worlds. It doesn't move water and air and plants around. The Traveler arrives at a place, Mercury for example, and it just forces it to have a suitable atmosphere and plants and liquids and to have whatever traits are necessary to preserve its adjustments. Our Guardian is more successful than others because their will and ability to shrug off doubts that would weaken their will surpasses others. That's why no other Guardians could have gotten their Light back from the Shard; they lack faith in their ability to act without the Traveler's Light. The Guardian was absolute in their certainty, and their will made it possible for Light to flow from the Shard.