IMO I'm using it correctly here though, let me explain.

Overhead is originally a business term that refers to an ongoing cost. Yes there is a small amount of code in the kernel that has to run, but my naive understanding is that this code also runs for processes that are not in a container (the kernel still needs to check whether the process is in a namespace or not). Additionally, I've never seen a benchmark that shows a workload performing worse when cgroups are applied. I'm happy to be proven wrong here but if this is the case, then there is no ongoing cost (and thus no overhead).