[conformist]

> “optimizing for the first derivative” played well with this approach

That's... still linear? If the concept of non-linearity is supposed to add anything to this article, arguably this should be at least the second derivative!

[svnt]

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).

[taneq]

Only if the first derivative is constant. ;)