Nature does nano-assembly, but it isn't arbitrary nano-assembly.

My example of a very hard nano-assembly problem is a ham sandwich, with the hardest part being the lettuce. It's possible that the easiest way to make a lettuce leaf-- they still have live cells-- is to grow a head of lettuce.

Maybe the right question (ignoring where MNT fits with AI) is to look at what parts of MNT looks feasible at present levels of knowledge.