Sending MMS with WiFi only iPad Air
That would be correct.

If you are trying to send a pic to your contact that is not on iMessages, then it would use the MMS system (the phone system) to deliver it.

That has little to do with WiFi but with how else should it deliver it without knowing the other persons WiFi connection. The iMessages would use the other person phone number as identification to deliver a MMS.

If you use any other communication system like Skype, Twitter, Facebook ..., both parties have to be on it, so the messages/pictures flow occurs using that app.