The information provided here is not a replacement for a doctor. You shouldn't use it for the purpose of self-diagnosing or self-medicating but rather so you can have a more informed discussion with a professional TCM practitioner.
Because it is a herb specifically indicated to treat pneumonitis as can be seen on Monk Fruit's page.
Its main actions are: Lubricates and cools the Lungs. Treats Lung Yin Deficiency. Dissipates nodules and swollen glands. Lubricates the Intestines and relieves constipation.