I don't know where you read that you could, but you cant. TPT uses Celsius for everything, I think it makes more sense to continue using it and not have an option. Fahrenheit would be meaningless for almost all TPT reactions anyway, it's good for weather but useless for almost every other application (once you get outside of normal air temperature ranges)

Oh, but in the PROP tool, you can set Fahrenheit using "100F" now, if you want.