The guy basically has been able to use parts of his visual cortex (usually used for visualizing) for hearing-based navigation and "imagining". This is because he learned to echolocate at a very young age; this is much harder for adult seeing-impaired folks to do.
I remember there was a brain imaging study he participated in which essentially showed "that brain structures that process visual information in sighted people process echo information in blind echolocation experts." He was "seeing" sound, essentially, in a way that gives him some amount of spatial resolution, much poorer than eyesight, but much better than what the traditionally seeing-impaired community has.
Visual processing is exhausting for me, so I rely on blurry peripheral vision or sound processing. I get way more information out of auditory information than my peers -- things like accurate 3D sound location, both types of materials in an impact (not just the louder sound), how close a stressed object is to breaking, how far away from a wall I am in the dark by the change in "sound shape" (sound being absorbed by the surface creating an echoey hum in the ears), high pitched noises (even at almost 40 I can hear high pitches like a teen), awareness of how my environment alters sound so I can still recognise sounds that are distorted, etc.