I suspect this is just using a MMS gateway on the internet.
In Spain there are similar apps. Basically Movistar has a website (espaciopersonal.movistar.es) that lets you send and receive MMS.
In fact when you receive an MMS instead of the cryptic username and password text you get in the US with AT&T, you simply receive a txt letting you know that you received a multimedia message, and links you to the espaciopersonal website.
There you log in with your tel number and password (one-time off registration) and browse all message just like if it were a webmail service.
So this MMS app we have here is simply an iphone native application that hooks itself to the espaciopersonal web. When you compose a mms, it is simply delivered thru that web.