In asbāb al-samāʾ bi-sullam, asbāb denotes means of ascent or paths, and sullam is a ladder. Zuhayr imagines an almost impossible escape route to show that even such a refuge cannot eliminate mortality.
The picture is an instance of hyperbole in the service of a general observation. Fear on earth is set against the widest imaginable upward flight, and the contrast intensifies the certainty of the consequence.