[claudiawerner]

>I figured it was some probably-from-critical-theory-because-aren't-they-always liberal-artsism, but looks like it's just an unusual-bordering-on-archaic sense.

I don't think that's correct (and the age of the dictionary quoted probably isn't helping); if you search Google Scholar for "knowledges", even since the year 2000 you get about 85,000 results. It's a term of art in sociology and anthropology fields, and marginally within philosophy too. It points to multiple understandings of the world which are disparate enough to be their own 'system' of knowledge; multiple such systems are known as knowledges.