Why?
Fuel smell, flames, most likely about 28-31t of fuel left in that wing alone (guestimate based on article). I'm sure the tower and RFF would have told the crew the wing was on fire. Who care if Pax get injured going down the slides. Better then everyone getting vaporized in an explosion.
Mrdeux
The flaps and LED's are not wet. Most liked fuel soaked or spray from some sort of leak. All the fuel is in the Amazon wing section and fuselage (centre tank). You can't see the wings from the cockpit but you do have the taxi camera. You can't see the wing tips from the taxi camera. Either way I'm sure cabin crew, tower and RFF informed the cockpit crew.