I would assume that "lightweight" in this case means that they share a single Linux kernel. Or that there is an emulation layer that maps the Linux Kernel API to macOS. In any case, I don't think that they are running a Linux kernel per container.
You don’t have to assume, the docs in the repo tell you that it does run a Linux kernel in each VM. It’s one container per VM.
"Lightweight" in the sense that the VM contains one static executable that runs the container, and not a full fledged Ubuntu VM (e.g. Colima).