Imagine an alternative ReLU which had a narrow curved section, to smooth out the discontinuity. Now it has a continuous derivative, but that gradient is still zero for values < 0. This flat region is the cause of dead neurons, because backprop multiplies the propagated error by the gradient, to update the weight - and if the gradient is 0, the result of the multiplication is 0 , and the neuron's weights do not get adjusted.

A derivative can be continuous, and zero.