They seem to do very little that SystemD cannot already do with services and overlayFS, with the added benefit of being already available on most systems.
Do some containers run outside a VM? Docker for example "uses operating-system-level virtualization to develop and deliver software in packages called containers."
I'd personally blame marketing-speak on using "virtualization" at all (unless they refer to their windows/mac offerings, which can run a Linux VM as the docker host, on which the containers are run), but I can see how one could also stretch a definition of virtualization in a way that covers container.
Sometimes containers are run in VMs, but they are almost defined as "do not require a full VM running an OS, but instead talk to the host kernel".
One could argue that the key of virtualization is that a piece of software is run in an environment that pretends to be something else than the actual base system. A VM hypervisor runs an operating system in a way that it looks like as it is running alone on a physical machine, with some fake devices. From inside a container, similarly the environment is fake: it can't see processes outside the container, it's view of the file system or devices is modified, and it looks as if the things in the container were the only things on that kernel.
I'm not sure that is true. I suspect that a great many containers are running in OSs that are in turn running in VMs on hosts in "cloud" structures, perhaps eclipsing the number that are running on an OS on bare metal.
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