Actually it hasn't been done before. That CVE required elevated privelages on the host already. They didn't break out of a jail.

They did break out of a jail, the vulnerability was used to gain arbitrary code execution in kernel mode, which was used to modify the cr_prison structure, thus performing a jailbreak.

Again, they already had elevated or root privs. If you've already got root privs nothing will save you.

They had root inside a jail, which isn't the same as being root outside the jail. To be able to gain arbitrary code execution in kernel mode from a jail is a security vulnerability.