Sulfur is an effective fungicide if applied at the correct rate. It is an effective fungicide when it is used against soil-borne diseases. It is effective against most plant diseases, but it is not an effective fungicide for diseases that attack the fruit or leaves of plants. It is not effective against fungal diseases that are caused by airborne spores, such as powdery mildew.
Sulfur is a component of many proteins, so it would be possible that it could kill off fungi, but the process is likely not that simple. Even if sulfur is absorbed in fungi, it would need to be in an active state to do damage.
Sulfur might kill fungi depending on the application that happens or the kind of fungi. If you have a mold infection you need to take care of it. But if you are applying sulfur to your garden soil it will not do anything.