I realize now how I didn't do a good job at expressing what I meant by that. I'm going to edit the article and re-word that statement.
The concept that I was trying to express is the following: the temperature of a color is a relative value, and it can be truly assessed only comparing it to the temperature of other colors.
The color wheel is a great tool to help compare colors because temperature naturally orders their placement around the wheel.
I look at yellow as the warmest color on the wheel.
The closest a color is to yellow, on either side, the warmest. The further away it is from yellow, the coolest that color is.
The warmest colors all have some yellow in them, while the coolest colors all have some blue in them. Other than pure red, pure red has neither blue or yellow, but it's considered in the warm family.