"Fascism is itself less 'ideological', in so far as it openly proclaims the principle of domination that is elsewhere concealed"

The quote recommends that Fascism is not an ideology in the traditional sense. Instead, it honestly reveals its desire for dominance and control, something that other political systems might keep concealed. Adorno is essentially saying that Fascism does not try to validate its power grab with some optimistic viewpoint, but rather blatantly seeks to control and rule over those who oppose it.