Alright I checked out the map.
He uses a trigger_hurt with burn or slowburn properties to deal the damage.
Graphically he uses env_steam, env_fire and env_firesource.
When you are damaged, somehow a custom screen overlay is applied to your screen that emulates the fire effect. It is very low resolution though, and your body isn't actually on fire. Still not sure how he does this. Also, the DPS does not last outside of the source.
So, no "real" fire but a way to fake it exists.