I am not 100% convinced by this. The matchup between their painting-based economic index (it's the first component from a PCA analysis, the data for each painting being a vector of pixel-counts for colours in each of 108 bins based on HSV) and GDP growth is pretty dubious, and in places where the two vary together the painting-based metric frequently changes several years before the allegedly-corresponding change in GDP growth.

They have ad hoc explanations for the divergences and try to make lemonade out of the lemons by claiming that their index reveals "higher-frequency fluctuations that traditional series smooth over" but I am willing to bet that if they had had to predict the divergences before doing the calculations they wouldn't have been able to.

I think this is probably mostly pareidolia.