Have a look down a 747 upper deck slide, you will see what he means. On the day it ain't gonna work.
I've looked down an A340-height slide (in a cabin sim) and shat myself, so no doubt I would have similar feelings at the top deck of an A380/747.
But, emergency egress from a 737 is frightening enough if the aircraft behind you is on fire and people are desperate to get out. In that sort of situation, I would take my chances going down the slide, no matter how uninviting it looked.
(And by the way, how many passengers willingly chuck themselves down huge, high slides at water parks day-in, day-out on their holidays? Some of those are near vertical, without any form of enclosure, hardly even a barrier.)