With the author's Corne keymap, wouldn't holding down the backspace key to access the "raise" layer start deleting text?

It's a 'tap-hold' key, so the "tap" functionality is activated separately from its "hold" functionality.

Though, in order to get the same effect as "hold down backspace", e.g. QMK lets you have "tap then hold" as a way of invoking that functionality.