From what I understand, SELinux can prevent copyfail, dirtyfrag and maybe fragnesia, although it might not always.

I presume you are referring to the GrapheneOS post/thread about this[0], although this implementation is not the same implementation we see on Fedora or Debian for example and it appears these distros were (and are) still vulnerable to this exploit, with the out of the box configuration of SELinux on these systems.