Badiou hit Deleuze hard with his review of THE FOLD, finding it to be the symptom of a failure of pluralism. Viewed from the perspective of pluralism THE FOLD is not a good book, embodying a return to Deleuze’s pre-Guattari style.
Strategically, Badiou was adroit in choosing this book to launch his polemic with Deleuze, as it is more classical in style and more totalising in content than his collaborations with Guattari, and so constituted a weaker target.
The concept of the “fold” plays only a minor role in WHAT IS PHILOSOPHY? Taken as a concept, it is too synchronic, it should rather be the diachronic: “to fold”, but that would perhaps not be a good title for a book.
For Badiou THE FOLD embodies all that he finds wrong with Deleuze’s philosophy: it is both too descriptive (phenomenological critique: it privileges interiority) and too unified (ontological criitique: it privileges organicity), valorising unity, presence, continuity, life.