Negative space is when the omission of color, art, or "content" creates a new image in and of itself. For example, the arrow pointing forward in the FedEx logo is an example of negative space.
I make the distinction in the cheatsheet so that if someone googles it later they're less likely to get the incorrect impression that all white space should form an image—or that it's common.
Negative space is white space, but not all white space is negative space.