Stallman acknowledges the he did not invent the term, and describes how the "L" was added to connote freedom in the free-as-in-speech sense, rather than type of freedom the "F" was typically associated with.
FOSS was already a term to denote the other types of open source license like MIT, that is why FLOSS was specifically coined, to not only say that there is a difference in the word free, but also to differentiate GNU OSS license from the others.
