The OED and Merriam-Webster (along with the wiki, and many more) agree that a 'ruler' is also a 'a straight strip or cylinder of plastic, wood, metal, or other rigid material, typically marked at regular intervals and used to draw straight lines or measure distances.', so your pedantry is a bit out of place.
A rule is a device for measuring length.
Shame the project homepage introduces this error. Language barriers are cross domain, not just cross culture.