Intelligence is a mystery. As soon as we can build a rational system that demonstrates a particular kind of intelligence, the mystery and perception of intelligence is lost.
Citation needed. We had machines playing other games at better-than-grandmaster level by that point. People talked about AI's struggles to play another perfect information game with very simple rules because it couldn't brute force parse every single permutation as an indication of how limited it was, not an indication that true intelligence was pruning search trees.
Previously using tools was a sign of intelligence until we discovered that other animals (even insects) do that. Then it was required that you had to make tools, until Jane Goodall showed us that other apes do that. Until AlphaGo we had this idea of particular games, always upping the size of the decision tree. And now it looks like we are in a crisis and just have this vaguely defined general intelligence, a term with quite a racist history, that nobody actually knows what means.
Me personally am of the opinion that we need to ditch the word intelligence in science and technology (it can remain in philosophy). It has all the same problems of a grand unifying theory in cosmology, and a whole host of more (see Stephen Jay Gould to discover those).
Calling it a mystery is being optimistic, rather, I would call it a misdirection.