I too am a pedant when it comes to using the word "literally" :)
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).
cgroups require code to implement. This is overhead. "Literally" is not the word you should be using. Perhaps "little" overhead or "minimal" overhead?