Yes, cgroups can be used to impose limits on resources; and from the outside looking in, can look very similar to setrlimit(). But from the inside of the kernel looking out (generally, the perspective taken by the linux-insides book), they are very different subsystems; I expect that cgroups will come in quite a bit later. If the book were discussing how to limit resources, sure it should mention cgroups; but it is discussing how things work in the kernel, not how to get the kernel to do certain things.