I hear you - its just that I dont think that it neccesarily follows that an AGI in whatever form will necesarily be unconstrained or free to improve itself. I also happen to think that the biological systems are in-fact rather special and hard to replicate - especially in terms of efficiency and resillience. Just because some implementation of a universal turing machine can simulate intelligence doesn't mean it can do it well enough to survive the real world.
Finally, to me, nuclear reactions are kind of the opposite of AGI: I think it's vastly easier to blow something up (increase entropy) than to create an intelligence capable of understanding and improving itself (decreasing entropy - possibly at an accelerating rate).