Of course it's a layer. It's a layer that itself is made up of 6 sublayers. This is not up for debate:

https://en.wikipedia.org/wiki/Neocortex

It clearly states:

> The six-layer cortex appears to be a distinguishing feature of mammals; it has been found in the brains of all mammals, but not in any other animals.

What it evolved out of is entirely irrelevant -- everything evolves out of something else in some fashion.

While the neocortex does have distinct layers, the neocortex itself is not “layered” around the rest of the brain - it’s deeply integrated all over the place with the rest of the brain and nervous system. There is no hierarchical relationship between the neocortex and different systems it integrates with.

The reptilian equivalent to the neocortex is the dorsal ventricular ridge which evolved separately and in parallel. This presents two problems to the hypothesis: first the much simpler DVR serves much of the same purpose as the neocortex which was completely unknown at the time and second the most interesting bird species (the smartest ones) often don’t have an equivalent structure at all. There isn’t even a clear relationship between intelligence, complexity, evolutionary age, etc. After 250 million years of evolution any similarities are accidents of random convergence.