Wolfram language is basically as open source as Mongo or Elastic. Which is to say, the source is open and they want the clout of being open source, but it isn't open source.
Less, I think; last I looked, Mongo and Elastic were actually publishing all the source code for the core of the application, albeit under questionable licenses, where Wolfram (in spite of the article's attempt at misdirection) lets you see the scripting language on top of the (C?) core code, which is totally unavailable.