There's so much that can go wrong with such reasoning, given that intelligence (even at the size of a galaxy of Dyson spheres) is not a perfect God, as to render such arguments irrelevant and entirely worthless. Furthermore there's enough ways how the non-orthogonality can hold, such as e.g. almost all intelligences with wrong moral systems crashing or failing to improve, that are not covered by 'converges'.
meta: Tendency to talk seriously about products of very bad reasoning really puts an upper bracket on the sanity of newcomers to LW. As is the idea that very bad argument trumps authority (when it comes to the whole topic).
What type of reasoning would you prefer to be used when talking about superintelligences?
Just a minor thought connected with the orthogonality thesis: if you claim that any superintelligence will inevitably converge to some true code of morality, then you are also claiming that no measures can be taken by its creators to prevent this convergence. In other words, the superintelligence will be uncontrollable.