chroot more or less isn't secure at all. All they're really saying is that Jails aren't built on top of chroot, but rather are a ground-up reimplementation motivated by improving chroot.
Ah! For some reason I thought jails were built on something similar to chroot at the kernel level, meaning that it also came with the same security caveats.
(to be more accurate, I knew that it was not "on top" in the same way as Linux containers are built on top of chroot. )
