I don't want to speak for Sean, but I would assume he meant it's not critical for the center to match the front left/right so long as they are neutral speakers and have a flat response. Sean's speakers are all Harman products which have roughly the same target response. You can mix and match those without problem. It would be a problem if the center had wildly different voicing than the left and right fronts.
Here is a measurement of the response of the Oberon 7:
It is OK, not perfect. They would have a bit softer off-axis sound than on-axis. A neutrally-voiced center speaker should play nice with those.