There is an enormous difference between the formal mathematical definition of "computable", and "able to be run by a computer that could be constructed in this universe". AIXI is computable in the mathematical sense of being written as a computer program that will provably halt in finitely many steps, but it is not computable in the sense of it being possible to run it, even by using all the resources in the observable universe optimally, because the runtime complexity of AIXI is astronomically larger than the universe is.