Open source is not Free software. The blog post is about how Blender is firmly behind the GPL and free software, which is in opposition to just having available source code without assured user freedoms.
To clarify: the article contains advice about how to fund operations for plugin developers. I doubt that it is viable for all plugin developers to fund their operations off of documentation and services. I am just saying that we should be honest and express that funding the development directly will be needed and probably the only way to get it is donations. That's really all I am saying here.
One example is I was recently messing around with Space Engineers code. Keen software gives opens it up but doesn't mean you can just download the code and get the game for free
On the legal side, free software and open source could very well be considered sinonyms (if that makes sense). The actual difference is one of attitude and end-goals.