It's a silly example but he does give a constraint. The one constraint is that the heat has to come from the candles. The purpose is to illustrate that even if one variable is constrained, there are other variables that may not be constrained, that you had assumed were static (altitude, pressure). It's just an example which illustrates a point of the article: try and think of something you had assumed was fixed and change that instead of working with the parameters you thought you had. Not a bad idea overall.