I have enough confidence in FreeBSD jails to not worry about that. If I found someone who could actually break out of a jail I would at the least buy them dinner anywhere, their choice. You can't modify a running kernel by direct access. And you can't load modules. So it would be a very interesting hack to see.
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.
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.
