iMessage is actually an instant messaging service, like Google Talk or AOL Instant Messenger, except iMessage is made by Apple and is only for Apple products. OnOnly an iPhone or an iPad with cellular it can send SMS messages as well, and can automatically associate phone numbers of registered iMessage users with iMessage accounts, so if you try to use the app to send a text to a phone number registered to an iPhone, it will send an iMessage instead. ItiPads cannot send SMS messages to non-Apple devices nor an iMessage to a phone number not registered with iMessage from a Wi-Fi-only device.
If you read the article at the support.apple URL carefully, you will note that only iPhones are able to send SMS. iPads (Air or 2) with cellular (LTE or 3G) will not send SMS. The cellular connection you get on iPads (with cellular) model is only to access the Internet, not a cellular phone.
