Why is quality (greatness) and the devil in the details? Is this a paradox or no?

It’s not a paradox. It’s really nothing when you think about it. Since everyone has different values regarding different things, how could one say that any one thing is in the details? It all comes down to subjective opinion.

They’re really saying the same thing: success comes from getting the details right (assuming that the basic plan is sound); failure comes from screwing up the details of even a sound plan.

It is two sides of the same coin. In other words, depending on how well you execute the details, you end up either with success or a devil of a mess.