Please correct me if I am wrong, as it is a while since I flew the Fokker 100, but the ice detection warning on the aircraft is a 'late' warning for ice build up on the wings/tailplane. In other words, in normal operations, the wing/tail de-ice should already have been switched on. I seem to remember that the engine anti-ice is selected on at 5 degrees C or below in moist air and certainly doesn't rely on the ice detection warning as a reminder to activate it.
What I am trying to say is that, given the conditions that morning, the engine anti-ice should have been selected on for most of the descent and the wing/tail anti-ice selected shortly before the ice warning.
Crew did an excellent job putting it down in one piece once the second engine lost power.