Not sure if it had been explicitly stated before in lore but The Nine are technically fourth dimensional beings. The Nine originally were the 9 missing crew members of the Exodus Green. So this means Bungie is building this narrative around the notion that Time is the fourth dimension. Now in real science we believe Time is one of many higher possible dimensions. But this will explain how those 9 human beings went from 3 dimensional beings to somehow being the “spirits” of the solar system. They’re just fourth dimensional now. For folks who have seen The Expanse, think about how the show ended with the Ring Entities being able to move through every atom in our universe. Time is but a moment and for these 4 dimensional beings malleable.