According to that article, the data they analyzed was API prices from LLM providers, not their actual cost to perform the inference. From that perspective, it's entirely possible to make "the cost of inference" appear to decline by simply subsidizing it more. The authors even hint at the same possibility in the overview:

> Note that while the data insight provides some commentary on what factors drive these price drops, we did not explicitly model these factors. Reduced profit margins may explain some of the drops in price, but we didn’t find clear evidence for this.