It is highly probable that the crew were talking to the fire fighters and the decision not to evacuate may have been based on information from the fire fighters outside the aircraft and in a position to see what was happening.
Many spoon drains and culverts in the grass at Changi, add to that rain and the ground may have been too soft for fire engines. In any event, a fire engine over tarmac is likely to be faster than one over the grass.
So much rubbish posted here, sadly some from pilots. Fuel dump with a fuel leak?