Doesn't SELinux do that (and more)?

selinux doesn't really provide anything like ProtectHome or PrivateTmp mentioned in the article. SELinux only does access control, while systemd can create new resources that are scoped to specific service.

The problem is the “more”. SELinux is extremely flexible and does what the configuration tells it to do. And it does not compose well. Want to point whateverd at /var/lib/whatever? Probably works if the distro packages are correct. Want to make /var/lib/whatever be a symlink? Probably does not do what you expect. Want to run a different daemon that accesses /var/lib/whatever or mount it into a container? Good luck. Want to run a second copy of the distro’s whateverd and point it at a different directory? The result depends on how the policy works.

And worst: want to understand what the actual security properties of your policy are? The answer is buried very, very deep.