I'm being a little unconventional with my terms - mostly because the article is titled "Open Source is Not About You." Zach Tellman (a prominent Clojure community member who does not work for Cognitect) just tweeted a link to his post
https://medium.com/@ztellman/standing-in-the-shadow-of-giant... about "open source" and mythmaking, which is a much subtler take (and, I'd guess, probably inspired by an older flareup in the Clojure community over governance).
It is, however, the case that open source under the GPL is a perfectly well-defined concept, as is free software under the MIT license. The terms refer to worldviews about the code and ethical obligations, not to licenses.