It is an interesting way of looking at the maximal potential of AIs. It could be that Oracle Machines are possible in this universe, but an AI built by humans cannot self-improve to that point because of the bound you are describing.
I feel that the phrasing "we have reached the upper bound on complexity" and later "can rise many orders of magnitude" gives a potentially misleading intuition about how limiting this bound is. Do you agree that this bound does not prevent us from building "paperclipping" AIs?
The Kolmogorov complexity ("K") of a string ("S") specifies the size of the smallest Turing machine that can output that string. If a Turing machine (equivalently, by the Church-Turing thesis, any AI) has size smaller than K, it can rewrite its code as much as it wants to, it won't be able to output S. To be specific, of course it can output S by enumerating all possible strings, but it won't be able to decide on S and output it exclusively among the options available. Now suppose that S is the source code for an intelligence strictly better than all those with complexity <K. Now, we are left with 3 options: