The best known example is that the CIA did in fact support nour al din al zinki, who were literally fighting side by side with nusra in Aleppo (and now in idli

The best known example is that the CIA did in fact support nour al din al zinki, who were literally fighting side by side with nusra in Aleppo (and now in idlib) in the same operation group. So you have various groups coordinating and working together with some light delineation between them. That's how technically the CIA can deny that they ever directly armed al qaeda.