Writing is not a garment, but a disguise.
His point is that often, particularly in French, a written word has absolutely no relationship to its phonetic representations--his example is the french oiseau ("bird") that does not have a single letter that corresponds to its pronunciation [wazo]. But I wonder if we can take this further a bit, and say that not just writing, but language is not a garment, but a disguise. By altering one's speech, one alters one's identity, others' perspective of you--rightly or wrongly about your gender, your class, your level of education, your geographic origins, your nationality. While speech may function as a garment, perhaps an imposed one, in socializing you, in letting others know particular information about you (much like clothing itself does), you can also use it to hide, to transform yourself, or, knowing how people perceive or preconceive particular speech patterns, to mislead.