That article describes this as a misunderstanding:
> The official definition of “open source software” (which is published by the Open Source Initiative and is too long to include here) was derived indirectly from our criteria for free software. [...]
> However, the obvious meaning for the expression “open source software”—and the one most people seem to think it means—is “You can look at the source code.” That criterion is much weaker than the free software definition, much weaker also than the official definition of open source. It includes many programs that are neither free nor open source.
> Since the obvious meaning for “open source” is not the meaning that its advocates intend, the result is that most people misunderstand the term.
That is, Stallman is criticizing "open source" for being, among other things, easy to misunderstand as not requiring free software licensing — even though it does, in fact, require free software licensing.