That is incorrect. Stallman defined 'free software'. He has said:
"The term “open source” software is used by some people to mean more or less the same category as free software. It is not exactly the same class of software: they accept some licences that we consider too restrictive, and there are free software licences they have not accepted. However, the differences in extension of the category are small: nearly all free software is open source, and nearly all open source software is free."
In particular, he notes the example of DRM software. DRM software can be distributed under an open-source licence, but by design and in practice it serves to restrict the freedoms of its users, not to enhance them.
"The term “open source” software is used by some people to mean more or less the same category as free software. It is not exactly the same class of software: they accept some licences that we consider too restrictive, and there are free software licences they have not accepted. However, the differences in extension of the category are small: nearly all free software is open source, and nearly all open source software is free."
In particular, he notes the example of DRM software. DRM software can be distributed under an open-source licence, but by design and in practice it serves to restrict the freedoms of its users, not to enhance them.