Zizek seems to be reiterating a criticism made against the adoption of mostly subjective attitudes. Here the historical source of the polemic seems to point back to Sartre's existentialism and an early comment made by Lacan in 1946.
Sartre's elaborated views are difficult to sumarise and when he attempted to present them to a large public most often a kind a caricature replaced them. As he sought to present a Kantian self sustaining morality without external support and constraints, sincerity (as opposed to bad faith) and dedication became central for it. A popular exposition was given in his lecture L'existentialisme est un humanisme in 1945 and published the following year). In 1946 Lacan published a Propos sur la causalite psychique which pointed to an obvious weakness in Sartre's position: a madman is sincere and dedicated in his existence. Lacan's text begins with mentioning that
il convient de remarquer que si un homme qui se croit un roi est fou,
un roi qui se croit un roi ne l'est pas moins. (..if a man, who
believes that he is king, is mad this is just as true for a king who
believes himself to be a king)
and a few lines later he writes a phrase with an easily recognizable sartrean wording and tone
le risque de la folie se mesure à l'attrait même des identifications
où l'homme engage à la fois sa vérité et son être. (the risk of
madness is measured by the attraction of identifications in which man
engages both his truth and being)