WSL2 distributions share the same Linux kernel. They only get their own root filesystem with a Linux userland (/bin, /usr, /lib etc), and some WSL config meta data. This is then stored as a virtual disk image (which is probably where your belief comes from). But the kernel runs in a single utility VM. The distros share that kernel instance and they are separated via namespaces only.

This makes running multiple WSL2 distributions in parallel very performant btw, as there is no world switch.