Misleading title. "Printer driver" suggests a piece of software which makes the printer accessible for other programs. The software in this article merely converts an image into a different format.
I suppose you're right... "CUPS Filter" might be a better word than "Driver", but I'm not sure most people would be familiar with that term before reading the full post. It would not be hard to add this script into CUPS as a filter so that it's accessible to all programs. At the end of the day, that's all that a driver is: a filter + a PPD, and the PPD already exists.