An apparent matter of perception . . . ?
The article sort of states the reasoning is that the human subject is already expecting a warm/hot red stone, though the red and blue may actually be the exact same temperature, and "believes" it is the cooler of the two, and vice versa with the blue stones, expecting cool, and "believing" it to be the warmer of the two.
Anyway, that was my take on it. I did not see where this was a blind, or better a double-blind test, perhaps holding stones inside a blind-box, or even as simple as behind one's back.