What I believe is occurring is this, Nezerec isn’t a physical entity but a metaphysical one, one that exists primarily on the Psychic level which is why even after his physical body was gone in Root he still is talking. What the witness did was provide a body for him to inhabit, we are aware that he actually grows in strength based off fear and pain which are mental not physical things. Its possible thats how he consumes and the more he consumes the stronger he gets. What I believe: The witness had Rhulk design a body for him as Rhulk also created the upended, glaives, and alot of pyramid tech as he was the first disciple. Rhulk created a body similar to his own as he believed he was the strongest being. It makes sense as Nezerec created the Nightmare defenses we see on the Moon. I think we will be seeing him again someday in another raid or dungeon that takes place in the mindscape that a few psions project us onto to finally end him. Its not too far fetched as in the lore for some of Calus shadows there was a race made of pure arc energy that he conquered showing races can evolve in certain ways. As well the Psions were conquered by a God Thought of some sort, and were dominated by it. They specifically say thoughts rather than a physical body. It would be a extremely interesting concept and would allow us to fight Nezerec once again.