Any label needs context, consider the label "3" or even "9 m/s^2," the first can refer to anything the second to an acceleration or to a gravitational field strength. In the case of weather, "extremely hot" is just what one would call a summer that is hotter above the usual variability. I would argue, that the label is rather good for the context of a NYT article in that the reader would refer to that category as "extremely hot."