Arffs major foam tenders have underbody sprays if needed to deal with burning fuel on approach, the smaller truck derived arffs appliances generally don't, depends on specification. Looking at the videos it was a substantial external fire at the rear, aluminium melts at a relatively low temperature, anyone who has experienced a jet A1 fire will appreciate how hot it is. The aim is to create surviveable conditions for the pax to get out, there was a point where the fire was pushed by a monitor towards R1, fortunately it didn't comprimise the chute. Had the arffs been at standby positions, would more have survived, we'll never know.