Yeah that's basically how it works, you have to use the # from JMP (which is mapped to an XMPP account) to receive/send MMS.
I'm using a similar setup to help test MMS stuff: https://blog.craftyguy.net/sendxmpp-mms/
I just didn't want to have to give out a second # to have people MMS me with, since that's annoying. So I'd rather see native MMS stuff working :D