That simply moves the problem. An agent such as AIXI if it worked would start by upgrading itself to beyond what its original programmers were able to design.
The important difference is that with this approach, we could never actually get to understand how powerful AI is "really" made, as we would be isolated from it by too many levels of abstraction.
Same as currently we train deep neural networks, and we get them to work (eg. for image recognition), but we don't really know - nor care - what rules and algorithms they developed for themselves under the hoods.