It sounds like he looped/threaded the midpoint of the rope around something and then rappelled on both strands. Once back in the building he could pull either end to retrieve the rope.
Possibly either loop the rope and descend on both ends - once inside retrieve by pulling on one end, alternately, use a knot that only holds under tension, once inside, shake rope to release knot, finally, it could be fantasy and the author didn't consider that detail.
A 70m rope (depending on the building) should manage 7 floors with a double strand (only just, and you'd want to use some webbing to anchor to the edge). The fact that he mentions a tibloc indicates he was rapping on a single strand though as that device would not handle a double strand ascension. I doubt I'd be comfortable anchoring to an AC unit though. Also, really, don't try this at home. There are lots of non-obvious hazards here. For instance, climbing ropes are incredibly strong and in practice, just don't break, unless, oh, say, they are weighted over a sharp edge like the corner of a building. And no, do not try the "shake the (insecure) knot free" technique, really. Rappelling is (statistically) very dangerous as it is one of the few situations in climbing where you are trusting your life to a single point of failure (redundancy is good! and is probably responsible for more deaths than climbing falls are [1]
Watch out! That technique is really sketchy, and won't work if the rope goes over an edge (like the edge of the building). You also have the problem of a falling metal object that can smash windows, your head, or itself.
