They don't make it clear at that point, but the section on the interrobang makes it clear we're talking about old typewriters. "[backspace]- would result in the - being directly underneath the " on a monospace typewriter.

̋-Like this̋- - backspacing on a typewriter let you do a primitive form of "composed characters"

Edit: it looked right in the submission box but not in the comment. Shrug.