Nobody is talking about every piece of software having to be free; if they want to make add-ons that can be sell they should have made those plugins for a software that allows such financing model such creating plugins for Autodesk Maya.
The statement in the article makes it sound as if providing documentation, support, etc. is a somewhat reliable way to get your work on Blender plugins and — implied — on other free and open-source software funded. I guess that's the reason for the impression that this somehow generalizes the statement to a broader software market.
"Your customers will love you!" even implies that this way of doing business is somehow superior to other licensing models.
I'd say: Your customers might love you, but for most projects this is likely also going to translate to "your customers won't pay you".
