I think you have it backwards. They weren't measuring humidity to calculate temperature. Rather, for home cooks, temperature is used to determine humidity.
In fudge making, the goal of cooking is simply to reduce the water in the sugar syrup to a specific concentration. Sugar syrups produce different hardness of candy at different concentrations.
When cooking other things- such as meat- the temperature is important because bad bugs are killed or meat proteins denatured at specific temperatures. Sugar, on the other hand, isn't changed at the temperatures used in candy making. Water is just boiling away. It's usually a bad thing when the temps get hot enough to start changing the sugar, ie caramelization.
Sugar syrups happen to boil at higher temperatures as they become more concentrated. Therefore the temperature can be used to determine the concentration. Temperature is much easier to measure than water concentration. Unfortunately, the boiling temperature of water changes dramatically depending on elevation. I moved from sea level to the mountains and the boiling point of water changed by 9 degrees F. That is a significant difference in candy concentration and I have to reduce the cooking time for my recipes.
In a commercial setting, it is worth the cost and effort to measure the syrup humidity directly. It's more foolproof. However, for home cooks, thermometers are cheap and easy to use. Therefore, most recipes are written in terms of temperature and not sugar concentration.