I've seen that same policy in regular corporations. Giving any member of a team 5/5 meant a personal phone call with HR and your skip level to justify it, while the message was sent unofficially beforehand that having that meeting would be bad for a manager's career.
when I had >8 direct reports I was usually allowed to give a single 5/5, greatly exceeds expectations, etc, but it was also made clear to me in unrecorded formats that I also better have someone with a failing score if I did that.