You should look at their paper. They have an impressive survey of prior approaches including Cyclone, and they claim that their approach (if I understand them correctly) could allow you to do some things outside of unsafe blocks if implemented in C# or Rust.
I don't see any use-after-free prevention here from a skim of the paper, so this doesn't seem to address many of the most important benefits you get from C# or Rust.
Use-after-free is not a theoretical problem. All of the Pwn2Own vulnerabilities this year were UAF, for example.
