In those datasets you could improve compression by adding apriori knowledge about their context in the real world - if the compression program knows about historical S&P500 prices, it will be pretty good at compressing all historical stock info. Kolmogorov complexity is indistinguishable from AI
They are not random (e.g. weather data should have a daily or seasonal pattern). And the fact that there does exist a random-looking data doesn't refute the original claim because AI can conclude that the data has a close-to-maximal entropy (i.e. "incompressible") instead.
https://news.ycombinator.com/item?id=38399753
https://news.ycombinator.com/item?id=31923231