If this was a haphazard set up of a zip line (with the only one qualified to actually set it up going rogue PS: Saul is still a bastard because now he might have caused Burt and Angel's deaths), it's quite possible that the rope just snapped on its own and dangled the 30 feet (10 yards) down into the top of the flames (or not which I'll cover in a second).

We know the adjacent tower is smaller than the Tower, so lets say it's 9 stories tall (under the assumption someone put forward that it would take 6 stories to zip line down to the roof and 6 more to zip to the 3rd floor of the Tower (per chapter 7), that means the rope was dangling 60 feet from the ground. If you assume the tanker was 12 feet tall, that means the distance is closer to 50 feet from the fire. And we know there is thick black smoke rising from that fire. And while smoke doesn't necessarily have to be as hot as the fire itself, it can still carry the heat. Fire doesn't have to touch an object to singe it. Extreme heat can accomplish the same task.