That makes sense even if you assume the NSA is adhering to its mission: if you don't publicly disclose the existence of differential cryptanalysis and the enemy

That makes sense even if you assume the NSA is adhering to its mission: if you don't publicly disclose the existence of differential cryptanalysis and the enemy hasn't figured it out the enemy may standardize on a weak cipher. The NSA wins in both of its missions: make the US more secure, make everyone else less secure.