RHEL6 starts upstarts, but doesn't make use of any of the functionality. Everything is sysvinit scripts and zero (upstart) jobs. I was administrating a few RHEL6 machines and had to read elsewhere about the change to even notice it.
The person you responded to is correct though; a large factor in starting systemd was the CLA of Upstart. Above even highlights this; you cannot really say that RHEL6 used upstart in any way.
I guess it depends on how you look at it. It uses upstart to start all the sysvinit scripts, no? I know you can create upstart jobs and they'll get started, I've done that. But you're right, it doesn't make "real" use of upstart, in a sense.
The person you responded to is correct though; a large factor in starting systemd was the CLA of Upstart. Above even highlights this; you cannot really say that RHEL6 used upstart in any way.