It depends on how you define 'security mechanism'

chroot has always provided security in that processes in a chroot in that it prevented that process from accessing any files outside that chroot.

This doesn't mean that this security couldn't be bypassed (if you could get root inside the chroot), but if you did not have root in the chroot, it helped.

for example, if a server had a vulerability that allowed it to access arbitrary files on the filesystem, putting it in a chroot can be very useful.