Since when is a security issue which is known to and patched by the vendor a "0-day"? Have there been any reported exploitations of this vulnerability in the wild before it was publicly disclosed with a patch made immediately available?
No, there were not, and it was disclosed to related vendors some weeks earlier. Some people seem to thing "0 day" is a l33t term for "vulnerability" it seems.
I think all they are suggesting is that running SELinux can protect you from vulnerabilities that no one knows about yet. Obviously they can't give an example of an vulnerability that is still a 0 day.
I think they mean "SELinux was stopping this vulnerability from being exploited before it was even discovered."