Bitumen or rubber-based membranes are flammable so it's the structure or substrate/ceiling underneath it that provides the passive fire protection because the vulnerability to the membrane is usually from underneath within the building and not above (if someone possibly left a heat source on top of the membrane).  With the project still under construction when the fire occurred, most of the passive fire spread prevention would have been in place but little of the active fire suppression (i.e. sprinklers) wouldn't have been.
Also, most passive fire protection measures aren't there to prevent a building from being destroyed but to let the occupants escape before the fire moves from one area (called a fire-cell) to another.  Most intertenancy walls are designed as 30/30/30 or 60/60/60 or 90/90/90 meaning it would take either 30 or 60 or 90 minutes for that fire element to have burnt enough for either the fire to pass through it into another fire area or, in the worst case, that amount of time for there to be a structural failure.
Fire on a roof isn't departmentalised into fire-cells so can spread quickly across the whole of the structure before it "finds" a weak point to burn downwards into the building.... and when the fire starts overnight, or during a lunch break, on the roof of a building before fire detection, prevention and passive fire spread items are in place, it can have disastrous consequences.