The "they," of course, refers to both Gilles Deleuze and Félix Guattari. While the text is, ostensibly, "about" Deleuze, much of his work, an undecidable amount of the thought comes from both writers. In order to refer to one, one must, in some sense, then, always refer to both. Cf. with this passage from A Thousand Plateaus :

"the two of us wrote Anti-Oedipus together. Since each of us was several,there was already quite a crowd"