I guess they are wrong. Even the recently updated list of the 160 handsets supporting W-AMR doesn't mention the N9. Stangely a "Nokia N7-00" is on the list...
It could be possible the N9 unofficially supports W-AMR. Would be nice if someone on those networks could confirm this. The sound quality should be very audible, especially with a (wired) headset. Kallela's comment earlier would make this highly unlikely as well though.