MS call it a "lightweight utility virtual machine", and "not a traditional VM experience", whatever that means.
Docker also works on MacOS, with a basic linux distro running under hypervisor that again, I'm not sure qualifies as a 'fully fledged VM' (at least in terms of user access to it).
WSL, starting with v2, is a fully fledged virtual machine, and comes with its own built in Linux kernel. The 'lightweight' part is because resource allocation is bit more dynamic than your usual VirtalBox VM. Other than that, there is no difference.