It this point in time it's indeed almost impossible to find out whether Bitcoin was created by an AI. We humans are too cocky when it comes to claiming first place in terms of intelligence. Granted, the human race is remarkably clever and resilient, but we also tend to think that we are the one and only master in writing code. If an AI would have emerged somewhere in our giant information technology infrastructure and it managed to stay undetected, it's clear that this entity would be better in writing code since code is it's natural environment. Sure, AI is not yet able to create great pieces of music, because that's typically a human thing, but that AI will outperform each and every programmer alive today.
So stating that AI did not create bitcoin is a sign of fear, fear that the AI actually might be a better programmer.