I can't find the chart you are referring to, so I'll take a guess:
The Airport Extreme supports 802.11a/b/g and the draft 'n' standard. I think what you saw by "compatible" referred only to the 802.11n compatibility, as the Core 2 Duos support it with the 802.11n enabler that comes with the AE. Any wireless computer (Mac or PC) that uses 802.11a/b/g should work with it.
Macs from the Core Duo line and earlier don't have 802.11n capability built in so they can't use the new standard, but will work using one of the earlier standards.
I think that answers the question. If I'm wrong, I'm sure somebody will chime in.
