Unfortunately as it is used in science and maths/engineering, non-linear popularly has two meanings: one is that it is continuous (the one you seem to be using)

Unfortunately as it is used in science and maths/engineering, non-linear popularly has two meanings: one is that it is continuous (the one you seem to be using), and one is that it is proportional: i.e. the derivative is fixed (which is what the author is using).